qemu: change macvtap device MAC address in response to NIC_RX_FILTER_CHANGED

qemu: change macvtap device MAC address in response to NIC_RX_FILTER_CHANGED

This patch fills in the functionality of
processNicRxFilterChangedEvent().  It now checks if it is appropriate
to respond to the NIC_RX_FILTER_CHANGED event (based on device type
and configuration) and takes appropriate action. Currently it checks
if the guest interface has been configured with
trustGuestRxFilters='yes', and if the host side device is macvtap. If
so, and the MAC address on the guest has changed, the MAC address of
the macvtap device is changed to match.

The result of this is that networking from the guest will continue to
work if the mac address of a macvtap-connected network device is
changed from within the guest, as long as trustGuestRxFilters='yes'
(previously changing the MAC address in the guest would break
networking).