Permit detecting seed-up on remote virsh VMs.

Remote VMs do not show up the local MAC table, but our design for them
has out of band IP allocation via Neutron so we can use ping instead
to detect when it has successfully DHCP'd.

Change-Id: Ie7273d1d4213e8a49d0bcf10ecb05ec7d47b8bec
Support remote VM hosts in boot-seed-vm.

With the upcoming test cluster, we need the ability to boot seed VMs
that are stored on a remote machine. This involves copying the image
onto the machine, chattr +C'ing it and then calling virsh start with
an appropriate URI.

Change-Id: I6194ad94d96aec08105f11acfae61b674a6f15c0