virtio-scsi going to be really a cool implementation on virtio stream. Indeed virtio-blk was arranging that position. How-ever virtio-blk lagged/skipped lots of scsi command ioctls which paved the way to virtio-scsi. Along with virtio-scsi implementation, other downsides of virtio-blk have been lifted, for ex: the number of disk limitation..etc.. To know more about virtio-scsi, I would like to ask you to go through below urls.
A better start would be watching a demo from the people behind it.. Slides of this doc can be found at:
http://www.linux-kvm.org/wiki/images/f/f5/2011-forum-virtio-scsi.pdf
Lets ..
Also refer below urls.
http://people.redhat.com/pbonzini/virtio-spec.pdf