bsn_in_port_128 OXM implementation

This extension OXM enables the controller to specify a bitmap of allowed input
ports. See the loxigen bsn_in_ports input file for more details.
OVSDriver: cleanup magic number and add comment
Merge into master from pull request #59:
bsn_in_ports_128 OXM implementation (https://github.com/floodlight/ivs/pull/59)