tools: virtio: introduce vhost_net_test

introduce vhost_net_test for both vhost_net tx and rx basing
on virtio_test to test vhost_net changing in the kernel.

Steps for vhost_net tx testing:
1. Prepare a out buf.
2. Kick the vhost_net to do tx processing.
3. Do the receiving in the tun side.
4. verify the data received by tun is correct.

Steps for vhost_net rx testing:
1. Prepare a in buf.
2. Do the sending in the tun side.
3. Kick the vhost_net to do rx processing.
4. verify the data received by vhost_net is correct.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>