Virtual drivers for windows

Hi,

This is more of a 'feature request' and I'm not expecting this pull-request to be merged as-is.

I have a requirement to install the virtio drivers into a windows system during install.
In order to do this I need to make available a second CD image - the one from RedHat which contains kvm's signed virtio drivers.

I've only extended the behaviour for virtual box but it should be relatively trivial to implement for other providers.

I'd suggest however that the whole ISO availability should be abstracted to a map with properties which include things like: download url, instructions, md5 hash / sha1 hash etc. just as is already present for the main OS ISO.

Let me know what you think

Brett