virtio_scsi: move to uapi

Guests need to use virtio scsi API, so export it to uapi,
nice to e.g. qemu and will help us remember this file
affects ABI.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>