Add GenerateMAC utility function

This adds a GenerateMAC auxiliary function for generating an unique MAC
address. It clears the LSB bit for "unicast" and sets the next-to-LSB
bit for "locally administered".

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
qemu: MAC address auto-generations

This patch adds "--mac" command line option for specifying the MAC
address of a QEMU instance. If it is not specified, the MAC address is
generated automatically.

This fixes multiple QEMU instances from receiving the same IP address
via DHCP.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>