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

[Richard W.M. Jones]

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.