netinfo: Add shorthand NetInfo.bridges for consistency

Change-Id: Ibf14be0a4db0678b4653512accd48b5ee9a43e1d
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25212
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>