Fix scripts/setup-vif-rules to work with VLAN-tagged networks

VLAN-tagged networks/ports are connected to a "fake bridge" device.
Most ovs-* commands will not work with a fake bridge
device, so this function needs to return the true parent bridge
device name, not the fake bridge.  Otherwise, subsequent calls
in this script to configure port locking via ovs-ofctl will
(silently) fail, and port locking will not activate.