Fix assert bug in testBrokenBridgelessNetReplacement

If the underlying device is missing, vdsm will not report
the network, but its config in vdsm is still there before
the broken network is removed.

Change-Id: Ie15ae32d83dbc771c675306b9aa8816162e9d7e3
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/22710
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>