Added support for setting custom MAC address to guests

Hi Chris,

I don't know if you'll find this option useful to be integrated in Oz, but I've implemented an argument to specify a custom MAC address for the guests.

```
oz-install file.tdl -m 00:00:00:00:00:00
```

This option was required to perform some customizations due to the restrictions of our network.

As always, if you like it and want me to improve it someway, let me know!

Cheers,

Luis