Istio VM expansion Use Case

I am trying to implement istio VM expansion usecase and i am trying to integrate bookinfo with a external mysql DB on my private cluster which is not on any cloud. I am facing issues in discovery of the VM, though istioctl registers the VM ip still their is no discovery the rating service can lookup to. The docs mention this is in development stage . and can be only implemented on GCP. Any thoughts on the above ? Plus any strong use cases of managing VM;s with istio ?