patch: Allow client to determine port names.

patch: Allow client to determine port names.

Calculating the patch port names from the bridge names makes sense when
there's only one pair of patch ports between a pair of bridges, but that
won't be the case for an upcoming use of patch ports.

This changes makes it easy to check for existing patch ports in
create_patch_port(), instead of in its caller, and since that seems like a
more sensible place this change also moves it there.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>