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>
Merge into master from pull request #36:
Add port_desc_stats processing support (https://github.com/floodlight/ivs/pull/36)