vboxnet host side MAC address should be configurable

[shinoda]

Host side MAC address of hostonlyifs, vboxnetX, are statically assigned 0a:00:27:00:00:X, where 0 <= X <= 127, which is good for the what it was designed for.

However, in certain applications of the virtualbox, these MAC addresses are announced to the external network as station behind L2 bridges. IEEE802.11s mesh networking protocol is an example of such application. In such cases, likelihood of MAC address collision is extremely large under the current assignment scheme. It should be fixed so that lower 3 octets could be configurable (just like the guest side of the interface).