ofproto/xlate: Add xlate_lookup_ofproto().

ofproto/xlate: Add xlate_lookup_ofproto().

"xlate_receive" did not tell much about what it is used for.  We have
two users of it that only want the ofproto and the OF port number, use the
new xlate_lookup_ofproto() for those.

Fix the comments of xlate_receive() as we no longer change the flow.

Also, the helper ofproto_dpif_contains_flow() seemed ill-named, so this
path removes it and uses xlate_lookup_ofproto() directly.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>