ofproto-dpif-xlate: Add xlate cache type XC_TABLE.

ofproto-dpif-xlate: Add xlate cache type XC_TABLE.

Xlate cache entry type XC_TABLE is required for the table stats
(number of misses and matches) to be correctly attributed.

It appears that table stats have been off ever since xlate cache was
introduced.  This was now revealed by a PACKET_OUT unit test case in a
later patch that checks for table stats explicitly.

Fixes: b256dc52 ("ofproto-dpif-xlate: Cache xlate_actions() effects.")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>