You can youse Number of VM's metric with ESXi dimension. But you will have to count changes od this value to assume that there was migration. I don't see other option to have this info from Dynatrace directly.
It seems that from 209already have it!
There are a few new events too. Cool stuff.
I need it in Managed, so I believe it will come with 1.210, which should be arround the corner, given it's already there for SaaS.
Noticed also that there are some other cool additions!