udev: net_id - support predictable ifnames on virtio buses

udev: net_id - support predictable ifnames on virtio buses

Virtio buses are undeterministically enumerated, so we cannot use them as a basis
for deterministic naming (see bf81e792f3c0). However, we are guaranteed that there
is only ever one virtio bus for every parent device, so we can simply skip over
the virtio buses when naming the devices.