datapath-windows: define mcgroup IDs for VPORT and other families

datapath-windows: define mcgroup IDs for VPORT and other families

dpif-linux.c makes a nl_lookup_genl_mcgroup(OVS_VPORT_FAMILY) that is not
implemented yet on Windows yet. Multicast group is used currently to
subscribe to events related to VPORTs. Whether the exact same mechanism
would be used is unclear yet.

In the interim, we can implement code to support nl_lookup_genl_mcgroup().
and make progress with the other simpler commands.

In this patch, we define a ID for the VPORT MC group and use it. The IDs for
other families were also defined, but without usage, were seen as unclear.
Hence we define only the VPORT group.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>