Booting with "systemd.log_level=debug" revealed:

Dec 18 17:26:22 ubuntu systemd-udevd[1326]: value '[dmi/id]sys_vendor' is 'QEMU'
Dec 18 17:26:22 ubuntu systemd-udevd[1326]: created db file '/run/udev/data/b7:2' for '/devices/virtual/block/loop2'
Dec 18 17:26:22 ubuntu systemd[1]: dev-loop2.device: Changed dead -> tentative
Dec 18 17:26:22 ubuntu systemd-udevd[1326]: created db file '/run/udev/data/b7:2' for '/devices/virtual/block/loop2'
Dec 18 17:26:22 ubuntu systemd-udevd[1326]: passed 293 byte device to netlink monitor 0x557a8c0f8680
Dec 18 17:26:22 ubuntu systemd[1120]: dev-loop2.device: Changed dead -> tentative
...
Dec 18 17:26:22 ubuntu systemd[1120]: sys-devices-virtual-block-loop2.device: Changed dead -> plugged
Dec 18 17:26:22 ubuntu systemd[1120]: dev-loop2.device: Changed tentative -> plugged
Dec 18 17:26:22 ubuntu systemd-udevd[529]: passed 179 byte device to netlink monitor 0x557a8c0c7bc0
Dec 18 17:26:22 ubuntu systemd[1]: sys-devices-virtual-block-loop2.device: Changed dead -> plugged
Dec 18 17:26:22 ubuntu systemd[1]: dev-loop2.device: Changed tentative -> plugged
...
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1/unit/dev_2dloop2_2edevice interface=org.freedesktop.DBus.Properties member=PropertiesChanged cookie=1009 reply_cookie=0 signature=sa{sv}as error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1/unit/dev_2dloop2_2edevice interface=org.freedesktop.DBus.Properties member=PropertiesChanged cookie=1010 reply_cookie=0 signature=sa{sv}as error-name=n/a error-message=n/a
...
Dec 18 17:26:22 ubuntu systemd-udevd[1328]: created db file '/run/udev/data/b7:2' for '/devices/virtual/block/loop2'
Dec 18 17:26:22 ubuntu systemd-udevd[1328]: created db file '/run/udev/data/b7:2' for '/devices/virtual/block/loop2'
Dec 18 17:26:22 ubuntu systemd-udevd[1328]: passed 252 byte device to netlink monitor 0x557a8c0f8680
Dec 18 17:26:22 ubuntu systemd[1]: sys-devices-virtual-block-loop2.device: Installed new job sys-devices-virtual-block-loop2.device/nop as 412
Dec 18 17:26:22 ubuntu systemd[1]: dev-loop2.device: Installed new job dev-loop2.device/nop as 413
Dec 18 17:26:22 ubuntu systemd[1]: sys-devices-virtual-block-loop2.device: Changed plugged -> dead
Dec 18 17:26:22 ubuntu systemd[1120]: sys-devices-virtual-block-loop2.device: Installed new job sys-devices-virtual-block-loop2.device/nop as 14
Dec 18 17:26:22 ubuntu systemd[1]: dev-loop2.device: Changed plugged -> dead
Dec 18 17:26:22 ubuntu systemd[1120]: dev-loop2.device: Installed new job dev-loop2.device/nop as 15
Dec 18 17:26:22 ubuntu systemd[1120]: sys-devices-virtual-block-loop2.device: Changed plugged -> dead
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1/unit/dev_2dloop2_2edevice interface=org.freedesktop.DBus.Properties member=PropertiesChanged cookie=1028 reply_cookie=0 signature=sa{sv}as error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1120]: dev-loop2.device: Changed plugged -> dead
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1/unit/dev_2dloop2_2edevice interface=org.freedesktop.DBus.Properties member=PropertiesChanged cookie=1029 reply_cookie=0 signature=sa{sv}as error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1120]: dev-loop2.device: Job dev-loop2.device/nop finished, result=done
Dec 18 17:26:22 ubuntu systemd[1120]: sys-devices-virtual-block-loop2.device: Job sys-devices-virtual-block-loop2.device/nop finished, result=done
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1/unit/sys_2ddevices_2dvirtual_2dblock_2dloop2_2edevice interface=org.freedesktop.DBus.Properties member=PropertiesChanged cookie=1030 reply_cookie=0 signature=sa{sv}as error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1120]: sys-devices-virtual-block-loop2.device: Collecting.
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1/unit/sys_2ddevices_2dvirtual_2dblock_2dloop2_2edevice interface=org.freedesktop.DBus.Properties member=PropertiesChanged cookie=1031 reply_cookie=0 signature=sa{sv}as error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1120]: dev-loop2.device: Collecting.
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1 interface=org.freedesktop.systemd1.Manager member=JobNew cookie=1032 reply_cookie=0 signature=uos error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1 interface=org.freedesktop.systemd1.Manager member=JobNew cookie=1033 reply_cookie=0 signature=uos error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1]: dev-loop2.device: Job dev-loop2.device/nop finished, result=done
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1 interface=org.freedesktop.systemd1.Manager member=JobRemoved cookie=1034 reply_cookie=0 signature=uoss error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1]: sys-devices-virtual-block-loop2.device: Job sys-devices-virtual-block-loop2.device/nop finished, result=done
Dec 18 17:26:22 ubuntu systemd[1]: Sent message type=signal sender=n/a destination=n/a path=/org/freedesktop/systemd1 interface=org.freedesktop.systemd1.Manager member=JobRemoved cookie=1035 reply_cookie=0 signature=uoss error-name=n/a error-message=n/a
Dec 18 17:26:22 ubuntu systemd[1]: sys-devices-virtual-block-loop2.device: Collecting.
Dec 18 17:26:22 ubuntu systemd[1]: dev-loop2.device: Collecting.