This patch provides the utility functions needed to synchronize
the rxfilter changes made to a guest domain with the corresponding
macvtap devices on the host:

* Get/set PROMISC flag
* Get/set ALLMULTI, MULTICAST
Signed-off-by: NTony Krowiak <akrowiak@linux.vnet.ibm.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>