virtio-net-test: add large tx buffer test

This test tries to build a packet whose size is greater than INT_MAX
which tries to trigger integer overflow in qemu_net_queue_append_iov()
which may result OOB.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20181204035347.6148-6-jasowang@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>