MCG paravirtualization support includes:
- Creating multicast groups by VFs, and keeping accounting of them
- Leaving multicast groups by VFs
- Updating SM only with real changes in the overall picture of MCGs status
- Creation of MGID=0 groups (let SM choose MGID)

Note that the MCG module maintains its own internal MCG object
reference counts.  The reason for this is that the IB core is used to
track only the multicast groups joins generated by the PF it runs
over.  The PF IB core layer is unaware of slaves, so it cannot be used
to keep track of MCG joins they generate.
Signed-off-by: Oren Duer <oren@mellanox.co.il>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>