teach delNetworks to remove a network entity under a bridge

By leaving the bridge intact, VDSM will be able to allow
partial network editing while VMs are connected to it.
Inter-VM networking on the bridge is also preserved during
network editing.
This commit does not yet change VDSM's behavior.

Change-Id: Ia6bb891f23ef4550f90c6c7b057aef6302cf435c
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35921
Reviewed-by: Dan Kenigsberg <danken@redhat.com>