arm: launch: libvirt: Use virtio-net-{pci,device} instead of hard-coding virtio-net-pci.

This is so it works on virtio-mmio platforms such as ARM.
tests: Add a regression test that ensure the network can be added.

This was only tested before (implicitly) by the rsync test in
tests/rsync.  It's better to have an explicit test of this
functionality.
arm: launch: direct: Use correct device name for virtio-scsi when detecting.