virtio-blk: fix comment for virtio_blk_rw_complete as nalloc is initially -1

The initial value of nalloc is -1, but not 1.
Signed-off-by: NDongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Message-id: 1541479952-32355-1-git-send-email-dongli.zhang@oracle.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/block/virtio-blk.c

@@ -96,7 +96,7 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
         trace_virtio_blk_rw_complete(vdev, req, ret);
 
         if (req->qiov.nalloc != -1) {
-            /* If nalloc is != 1 req->qiov is a local copy of the original
+            /* If nalloc is != -1 req->qiov is a local copy of the original
              * external iovec. It was allocated in submit_requests to be
              * able to merge requests. */
             qemu_iovec_destroy(&req->qiov);