xenserver: Accept VLAN PIFs in pif_bridge_name().

xenserver: Accept VLAN PIFs in pif_bridge_name().

pif_bridge_name() was written when the vswitch interface-reconfigure was
working with the raw Open vSwitch configuration via ovs-cfg-mod.  Then,
it made no sense to ask for the "bridge name" of a VLAN PIF, because a
VLAN PIF didn't have a bridge.

Now, however, the vSwitch interface-reconfigure works with ovs-vsctl, which
provides a thin layer over the OVS configuration that makes VLAN bridges
appear to exist.  So now it makes sense to ask for the bridge of a VLAN
PIF, and this commit enables that.

Signed-off-by: Ben Pfaff <blp@nicira.com>