scsi-disk: fix the mode data length field returned by the MODE SENSE command

The MODE DATA LENGTH field indicates the length in bytes of the following
data that is available to be transferred. The mode data length does not include
the number of bytes in the MODE DATA LENGTH field.
Signed-off-by: NBernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/scsi-disk.c

@@ -653,7 +653,7 @@ static int scsi_disk_emulate_mode_sense(SCSIRequest *req, uint8_t *outbuf)
     }
 
     buflen = p - outbuf;
-    outbuf[0] = buflen - 4;
+    outbuf[0] = buflen - 1;
     if (buflen > req->cmd.xfer)
         buflen = req->cmd.xfer;
     return buflen;