ovs-vsctl: Prevent creating duplicate VLAN bridges.

ovs-vsctl has the concept of a VLAN (or "fake") bridge, which is a
sort of a sub-bridge that receives only packets on a particular VLAN.
There is no way to distinguish two VLAN bridges with the same parent on the
same VLAN, but until now ovs-vsctl did not prevent creating duplicates or
report them.  This commit fixes the problem.

Reported-by: rwxybh
Reported-at: https://github.com/openvswitch/ovs/issues/21
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Thomas Graf <tgraf@noironetworks.com>