Add port_desc_stats processing support

Add support port_desc_stats_request in indigo,
which is typically Openflow 1.3 OFPT_MULTIPART_REQUEST
with MFPMP_PORT_DESC.

Signed-off-by: Hiroaki KAWAI <kawai@stratosphere.co.jp>
OFStateManager: add stub for indigo_port_desc_stats_get to utest

This fixes a build failure.