Fix bsn_vport subtype and add l2gre vport

Reviewer: @rlane
CC: @kenchiang

All the changes in c_gen/c_test_gen.py are there to handle testing of the of_bsn_virtual_port_create_request message with a concrete bsn_vport subtype (q_in_q).

Changes in c_gen/codegen.py are necessary for proper interpretation of the object_id for each bsn_vport subtype.