virtio: don't mark unaccessed memory as dirty

offset of accessed buffer is calculated using iov_length, so it
can exceed accessed len. If that happens
math in len - offset wraps around, and size becomes wrong.
As real value is 0, so this is harmless but unnecessary.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio.c

@@ -241,7 +241,7 @@ void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
                                   elem->in_sg[i].iov_len,
                                   1, size);
 
-        offset += elem->in_sg[i].iov_len;
+        offset += size;
     }
 
     for (i = 0; i < elem->out_num; i++)