util: Functions to update host network device's multicast filter

util: Functions to update host network device's multicast filter

This patch provides the utility functions to needed to synchronize the
changes made to a guest domain network device's multicast filter
with the corresponding macvtap device's filter on the host:

* Get/add/remove multicast MAC addresses
* Get the macvtap device's RX filter list

Signed-off-by: Tony Krowiak <akrowiak@linux.vnet.ibm.com>
Signed-off-by: Laine Stump <laine@laine.org>