Remove hard-coded virbr0.
Remove hard-coded virbr0.

Instead, do a search through the libvirt networks, looking for
the first NAT one that we can use.  This should allow oz to
work with non-standard setups.

Signed-off-by: Chris Lalancette <60b62644009db6b194cc0445b64e9b27bb26433a@redhat.com>

mgagne/oz,nullr0ute/oz,mgagne/oz,cernops/oz,imcleod/oz,NeilBryant/oz,NeilBryant/oz,clalancette/oz,moofrank/oz,ndonegan/oz,ndonegan/oz,nullr0ute/oz,clalancette/oz,imcleod/oz,cernops/oz,moofrank/oz