vrrp: Support sending VRRP packets from base VMAC interface.

This provides a new option to use in conjunction with the VMAC
functionality which will result in VRRP advertisements being sent over
the underlying interface (and therefore having the source MAC of that
interface rather than that of the created VMAC interface)

With this new functionality enabled, VRRP messages will not affect the
switch MAC address table since the non-unique VMAC address is now used
only for sending a gratuitous ARP, thereby ensuring that in conditions
of VRRP message loss, a probing partner will not inadvertently take over
traffic.

Signed-off-by: Oliver Smith <oliver@8.c.9.b.0.7.4.0.1.0.0.2.ip6.arpa>