virtio-win cleanups, updates and virtio-scsi support

Thanks to guest-hw.cfg, we can now enable virtio automatically when the "right" hardware combinations are present in the tuple.  This patch series does this, and also updates virtio-win.cfg for:

* new paths in virtio-win.iso and updates for new releases

* asset path in shared/downloads/virtio-win.ini

* virtio-scsi support

Floppy support still requires manual intervention.