tests: add /vhost-user/flags-mismatch test

Check that qemu disconnects the backend that doesn't have the previously
acked features.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
