configNetwork: Fix bridgeless broken network deletion

When trying to delete a broken bridgeless vdsm network, if the iface
referenced by the network definition was missing, delNetwork would
not get any device on its call for vlan, nic and bonds, but
objectivize would be called nonetheless, failing with a:

ERR_BAD_PARAMS: 'Network defined without devices.'

The fix consists on just removing the network from libvirt when the
backing device is missing.

Change-Id: I92c641d91da9ef7eab876ed8c77fff7a9becb503
Bug-Url: https://bugzilla.redhat.com/1020356
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22354
Reviewed-by: Dan Kenigsberg <danken@redhat.com>