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>
netlink-socket: add support for nl_lookup_genl_mcgroup()

While we work out whether nl_sock_join_mcgroup() will be the mechanism
to support VPORT events, it is easy to add support for
nl_lookup_genl_mcgroup() and make progress on the other commands.

In this patch, we implement support for nl_lookup_genl_mcgroup() only
for the VPORT family though, which is all what dpif-linux.c needs.

Validation:
- A ported dpif-linux.c with epoll code commented out went so far as
to call dp_enumerate! DP Dump commands can be implemented next.

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