net: vmxnet: initialise local tx descriptor

In Vmxnet3 device emulator while processing transmit(tx) queue,
when it reaches end of packet, it calls vmxnet3_complete_packet.
In that local 'txcq_descr' object is not initialised, which could
leak host memory bytes a guest.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
