virtio-blk: deprecate SCSI passthrough

The Linux virtio_blk.ko guest driver is removing legacy SCSI passthrough
support.  Deprecate this feature in QEMU too.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-id: 20191213144626.1208237-1-stefanha@redhat.com
Message-Id: <20191213144626.1208237-1-stefanha@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qemu-deprecated.texi

@@ -278,6 +278,17 @@ spec you can use the ``-cpu rv64gcsu,priv_spec=v1.9.1`` command line argument.
 
 @section Device options
 
+@subsection Emulated device options
+
+@subsubsection -device virtio-blk,scsi=on|off (since 5.0.0)
+
+The virtio-blk SCSI passthrough feature is a legacy VIRTIO feature.  VIRTIO 1.0
+and later do not support it because the virtio-scsi device was introduced for
+full SCSI support.  Use virtio-scsi instead when SCSI passthrough is required.
+
+Note this also applies to ``-device virtio-blk-pci,scsi=on|off'', which is an
+alias.
+
 @subsection Block device options
 
 @subsubsection "backing": "" (since 2.12.0)