Add '--force-virtio' parameter to oz-install

Hi Chris,

As we discussed previously on aeolus-devel, I'd find interesting to have the possibility of forcing Oz to use virtio drivers.

This allows users to take advantage of virtio with guests that weren't expected to be supported by virtio drivers. In this case, it's responsibility of the user to provide the adequate OS images.

I know you're busy these days, but I think it's easy to keep track of the proposal through github rather than the mailing list, isn't it?

Thanks :)