I updated the hosts libvirt to the version of the backport ppa you mentioned, inserted, delegated the USB-NIC and inserted the cable. It shows the same error as Ubuntu 22.04 before. This strongly suggests, that it is indeed a libvirt bug.

Host:
libvirt0:amd64/focal 8.0.0-1ubuntu7.1~backport20.04.202205310636~ubuntu20.04.1 uptodate


On Host:
dmesg:
[  446.125558] usb 2-4: new SuperSpeed USB device number 3 using xhci_hcd
[  446.145912] usb 2-4: New USB device found, idVendor=17ef, idProduct=7205, bcdDevice=30.00
[  446.145924] usb 2-4: New USB device strings: Mfr=1, Product=2, SerialNumber=6
[  446.145927] usb 2-4: Product: Thinkpad USB LAN
[  446.145929] usb 2-4: Manufacturer: Lenovo
[  446.145931] usb 2-4: SerialNumber: 1C6955000000
[  446.173154] usbcore: registered new interface driver r8152
[  446.183791] usbcore: registered new interface driver cdc_ether
[  446.302087] usb 2-4: reset SuperSpeed USB device number 3 using xhci_hcd
[  446.349809] r8152 2-4:1.0: load rtl8153a-3 v2 02/07/20 successfully
[  446.383514] r8152 2-4:1.0 eth0: v1.12.12
[  446.424504] r8152 2-4:1.0 enx3c18a01c6955: renamed from eth0
[  470.390919] audit: type=1400 audit(1663236916.760:125): apparmor="STATUS" operation="profile_replace" profile="unconfined" name="libvirt-21d2ab6e-d039-4e13-8956-4249b580d756" pid=18610 comm="apparmor_parser"
[  471.097459] usb 2-4: reset SuperSpeed USB device number 3 using xhci_hcd
[  471.151505] r8152 2-4:1.0: load rtl8153a-3 v2 02/07/20 successfully
[  471.179161] r8152 2-4:1.0 eth0: v1.12.12

journalctl:
Sep 15 12:14:52 airi kernel: usb 2-4: new SuperSpeed USB device number 3 using xhci_hcd
Sep 15 12:14:52 airi kernel: usb 2-4: New USB device found, idVendor=17ef, idProduct=7205, bcdDevice=30.00
Sep 15 12:14:52 airi kernel: usb 2-4: New USB device strings: Mfr=1, Product=2, SerialNumber=6
Sep 15 12:14:52 airi kernel: usb 2-4: Product: Thinkpad USB LAN
Sep 15 12:14:52 airi kernel: usb 2-4: Manufacturer: Lenovo
Sep 15 12:14:52 airi kernel: usb 2-4: SerialNumber: 1C6955000000
Sep 15 12:14:52 airi kernel: usbcore: registered new interface driver r8152
Sep 15 12:14:52 airi kernel: usbcore: registered new interface driver cdc_ether
Sep 15 12:14:52 airi kernel: usb 2-4: reset SuperSpeed USB device number 3 using xhci_hcd
Sep 15 12:14:52 airi kernel: r8152 2-4:1.0: load rtl8153a-3 v2 02/07/20 successfully
Sep 15 12:14:52 airi kernel: r8152 2-4:1.0 eth0: v1.12.12
Sep 15 12:14:52 airi networkd-dispatcher[867]: WARNING:Unknown index 20 seen, reloading interface list
Sep 15 12:14:52 airi systemd-udevd[18057]: Using default interface naming scheme 'v245'.
Sep 15 12:14:52 airi systemd-udevd[18057]: ethtool: autonegotiation is unset or enabled, the speed and duplex are not writable.
Sep 15 12:14:52 airi kernel: r8152 2-4:1.0 enx3c18a01c6955: renamed from eth0
Sep 15 12:14:52 airi systemd-networkd[513]: eth0: Interface name change detected, eth0 has been renamed to enx3c18a01c6955.
Sep 15 12:14:52 airi networkd-dispatcher[867]: WARNING:Unknown index 20 seen, reloading interface list
Sep 15 12:14:52 airi systemd-udevd[18059]: Using default interface naming scheme 'v245'.
Sep 15 12:14:52 airi systemd-udevd[18059]: ethtool: autonegotiation is unset or enabled, the speed and duplex are not writable.
Sep 15 12:14:55 airi ModemManager[887]: <info>  [base-manager] couldn't check support for device '/sys/devices/pci0000:00/0000:00:15.0/usb2/2-4': not supported by any plugin
Sep 15 12:15:01 airi CRON[18230]: pam_unix(cron:session): session opened for user rk by (uid=0)
Sep 15 12:15:01 airi CRON[18231]: pam_unix(cron:session): session opened for user rk by (uid=0)
Sep 15 12:15:01 airi CRON[18232]: (rk) CMD (/usr/bin/wget -6 --no-check-certificate --output-document=/dev/null --timeout=600 -q "https://naoko.zsze.de")
Sep 15 12:15:01 airi CRON[18233]: (rk) CMD (/usr/bin/wget -4 --no-check-certificate --output-document=/dev/null --timeout=600 -q "https://naoko.zsze.de")
Sep 15 12:15:01 airi CRON[18231]: pam_unix(cron:session): session closed for user rk
Sep 15 12:15:01 airi CRON[18230]: pam_unix(cron:session): session closed for user rk
Sep 15 12:15:14 airi systemd[1]: Starting Checkmk agent (10.91.0.249:38112)...
Sep 15 12:15:16 airi audit[18610]: AVC apparmor="STATUS" operation="profile_replace" profile="unconfined" name="libvirt-21d2ab6e-d039-4e13-8956-4249b580d756" pid=18610 comm="apparmor_parser"
Sep 15 12:15:16 airi kernel: audit: type=1400 audit(1663236916.760:125): apparmor="STATUS" operation="profile_replace" profile="unconfined" name="libvirt-21d2ab6e-d039-4e13-8956-4249b580d756" pid=18610 comm="apparmor_parser"
Sep 15 12:15:17 airi systemd[1]: check-mk-agent@6-10.91.0.203:6556-10.91.0.249:38112.service: Succeeded.
Sep 15 12:15:17 airi systemd[1]: Finished Checkmk agent (10.91.0.249:38112).
Sep 15 12:15:17 airi kernel: usb 2-4: reset SuperSpeed USB device number 3 using xhci_hcd
Sep 15 12:15:17 airi kernel: r8152 2-4:1.0: load rtl8153a-3 v2 02/07/20 successfully
Sep 15 12:15:17 airi kernel: r8152 2-4:1.0 eth0: v1.12.12
Sep 15 12:15:17 airi networkd-dispatcher[867]: WARNING:Unknown index 21 seen, reloading interface list
Sep 15 12:15:17 airi networkd-dispatcher[867]: ERROR:Unknown interface index 21 seen even after reload
Sep 15 12:15:17 airi networkd-dispatcher[867]: WARNING:Unknown index 21 seen, reloading interface list
Sep 15 12:15:17 airi networkd-dispatcher[867]: ERROR:Unknown interface index 21 seen even after reload
Sep 15 12:15:17 airi systemd-udevd[18653]: eth0: Failed to get link config: No such device
Sep 15 12:15:17 airi ModemManager[887]: <info>  [base-manager] port eth0 released by device '/sys/devices/pci0000:00/0000:00:15.0/usb2/2-4'
Sep 15 12:15:17 airi ModemManager[887]: <info>  [base-manager] couldn't check support for device '/sys/devices/pci0000:00/0000:00:15.0/usb2/2-4': Operation was cancelled
Sep 15 12:16:01 airi CRON[20061]: pam_unix(cron:session): session opened for user rk by (uid=0)
Sep 15 12:16:01 airi CRON[20062]: pam_unix(cron:session): session opened for user rk by (uid=0)
Sep 15 12:16:01 airi CRON[20063]: (rk) CMD (/usr/bin/wget -6 --no-check-certificate --output-document=/dev/null --timeout=600 -q "https://naoko.zsze.de")
Sep 15 12:16:01 airi CRON[20064]: (rk) CMD (/usr/bin/wget -4 --no-check-certificate --output-document=/dev/null --timeout=600 -q "https://naoko.zsze.de")
Sep 15 12:16:01 airi CRON[20062]: pam_unix(cron:session): session closed for user rk
Sep 15 12:16:01 airi CRON[20061]: pam_unix(cron:session): session closed for user rk
Sep 15 12:16:17 airi systemd[1]: Starting Checkmk agent (10.91.0.249:38114)...
Sep 15 12:16:20 airi systemd[1]: check-mk-agent@7-10.91.0.203:6556-10.91.0.249:38114.service: Succeeded.
Sep 15 12:16:20 airi systemd[1]: Finished Checkmk agent (10.91.0.249:38114).
Sep 15 12:17:01 airi CRON[21946]: pam_unix(cron:session): session opened for user root by (uid=0)
Sep 15 12:17:01 airi CRON[21948]: pam_unix(cron:session): session opened for user rk by (uid=0)
Sep 15 12:17:01 airi CRON[21949]: (root) CMD (   cd / && run-parts --report /etc/cron.hourly)
Sep 15 12:17:01 airi CRON[21947]: pam_unix(cron:session): session opened for user rk by (uid=0)
Sep 15 12:17:01 airi CRON[21950]: (rk) CMD (/usr/bin/wget -6 --no-check-certificate --output-document=/dev/null --timeout=600 -q "https://naoko.zsze.de")
Sep 15 12:17:01 airi CRON[21951]: (rk) CMD (/usr/bin/wget -4 --no-check-certificate --output-document=/dev/null --timeout=600 -q "https://naoko.zsze.de")
Sep 15 12:17:01 airi CRON[21946]: pam_unix(cron:session): session closed for user root
Sep 15 12:17:02 airi CRON[21948]: pam_unix(cron:session): session closed for user rk
Sep 15 12:17:02 airi CRON[21947]: pam_unix(cron:session): session closed for user rk

Nothing happend in /var/log/libvirt/qemu/management.log


On VM:
dmesg:
[  284.845919] usb 2-4: new high-speed USB device number 3 using ehci-pci
[  285.004095] usb 2-4: config 1 interface 0 altsetting 0 bulk endpoint 0x81 has invalid maxpacket 1024                                                  
[  285.007834] usb 2-4: config 1 interface 0 altsetting 0 bulk endpoint 0x2 has invalid maxpacket 1024                                                   
[  285.020715] usb 2-4: config 2 interface 1 altsetting 1 bulk endpoint 0x81 has invalid maxpacket 1024                                                  
[  285.024468] usb 2-4: config 2 interface 1 altsetting 1 bulk endpoint 0x2 has invalid maxpacket 1024                                                   
[  285.036259] usb 2-4: New USB device found, idVendor=17ef, idProduct=7205, bcdDevice=30.00                                                             
[  285.039715] usb 2-4: New USB device strings: Mfr=1, Product=2, SerialNumber=6                                                                         
[  285.042661] usb 2-4: Product: Thinkpad USB LAN
[  285.044804] usb 2-4: Manufacturer: Lenovo
[  285.048580] usb 2-4: SerialNumber: 1C6955000000
[  285.316969] usb 2-4: can't set config #2, error -32