Allow custom mac address assignment for IPv6 addresses. Some network providers require mac assignments for individual IPv6 addresses, and this currently requires manual work every time you want to assign an IPv6 to a KVM VM currently.

IPv6 and IPv4 should be allowed to be paired in this instance, since IPv4 and IPv6 mac address would need to be the same for the individual VM. If IPv6 have a custom mac address, and are assigned automatically, this could cause mix ups.