gem_transmit copies a packet from guest into an tx_packet[2048]
array on stack, with size limited by descriptor length set by guest. If
guest is malicious and specifies a descriptor length that is too large,
and should packet size exceed array size, this results in a buffer
overflow.
Reported-by: 刘令 <liuling-it@360.cn> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Jason Wang <jasowang@redhat.com>