tools/virtio: creating pipe assertion in vringh_test

parallel_test() function in vringh_test needs to verify
the creation of the guest/host pipe.

Signed-off-by: Yunseong Kim <yskelg@gmail.com>
Message-Id: <20240624174905.27980-2-yskelg@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>