virtio-scsi: Call virtio_add_queue_aio

virtio-scsi: Call virtio_add_queue_aio

AIO based handler is more appropriate here because it will then
cooperate with bdrv_drained_begin/end. It is needed by the coming
revert patch.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>