Comment 10 for bug 1883009

In your case it finds the device and tries to follow:
        Following Set Member: sys-devices-pci0000:00-0000:00:08.0-virtio2-virtio\x2dports-vport2p1.device
        Following Set Member: dev-vport2p1.device
It knows it is udev related
        Found: found-udev
But your path is different:
        Sysfs Path: /sys/devices/pci0000:00/0000:00:08.0/virtio2/virtio-ports/vport2p1

You miss (the following is only in my log) to reach:
        Wants: qemu-guest-agent.service (origin-udev)
        References: qemu-guest-agent.service (origin-udev)
        ...
        udev SYSTEMD_WANTS: qemu-guest-agent.service

It seems you have multiple virtio-serial ports.
Yours is "vport2p1"?
What is the first one?
Maybe an extra console?
Mine looks like:
$ ll /dev/virtio-ports/
total 0
drwxr-xr-x 2 root root 60 Jun 16 05:19 ./
drwxr-xr-x 18 root root 3920 Jun 16 05:19 ../
lrwxrwxrwx 1 root root 11 Jun 16 05:19 org.qemu.guest_agent.0 -> ../vport1p1

Hmm ... proxmox ... maybe that is something special in the setup and that is why you see it but no one else so far?

Do you have a guest XML of libvirt in the host that you can share so one can try to set up the same over here?