9p/trans_virtio: discard zero-length reply

When a 9p request is successfully flushed, the server is expected to just
mark it as used without sending a 9p reply (ie, without writing data into
the buffer). In this case, virtqueue_get_buf() will return len == 0 and
we must not report a REQ_STATUS_RCVD status to the client, otherwise the
client will erroneously assume the request has not been flushed.

Cc: stable@vger.kernel.org
Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>