[Libguestfs] v2v: -i libvirtxml: Map empty network or bridge name to a default (RHBZ#1257895).

When importing from VMware via the libvirt driver, the libvirt driver can add an empty source bridge name: <interface type='bridge'> <mac address='00:01:02:03:04:05:06'/> <source bridge=''/> </interface> Replicate what we do on the -i ova path, and map these to "eth0", "eth1" etc. This also includes a bunch of changes to how we do xpath parsing to make it type-safe. Rich.