vhost/test: fix memory leak of vhost virtqueues

We need free the vqs in .release(), which are allocated in .open().

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Link: https://lore.kernel.org/r/20211228030924.3468439-1-xianting.tian@linux.alibaba.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>