net: vmxnet3: avoid memory leakage in activate_device

Vmxnet3 device emulator does not check if the device is active
before activating it, also it did not free the transmit & receive
buffers while deactivating the device, thus resulting in memory
leakage on the host. This patch fixes both these issues to avoid
host memory leakage.

Reported-by: Qinghao Tang <luodalongde@gmail.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
