fix virtio_blk_inhdr comment

The inhdr is at the end of the S/G list, not the beginning.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-blk.h

@@ -67,7 +67,7 @@ struct virtio_blk_outhdr
 #define VIRTIO_BLK_S_IOERR      1
 #define VIRTIO_BLK_S_UNSUPP     2
 
-/* This is the first element of the write scatter-gather list */
+/* This is the last element of the write scatter-gather list */
 struct virtio_blk_inhdr
 {
     unsigned char status;