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: N刘令 <liuling-it@360.cn>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>