Fix for host systemd integration as config has gone and we're not using that for checking existance anyway these days.
Change to networking withing guest. Turns out that you *cannot* set the guest-side name of a veth device. That is a bummer. However, if is guaranteed that they are named ethNNN from eth0 and that the name will match the order in the XML. The network.nix needs a rewrite anyway, but it does now work (eg router, which needs 2 nics, does work just fine now).