Hi Thomas,
I’d start reading this old forum post (https://forum.opennebula.io/t/automatically-restart-vms-after-host-restart/454) which discussed this issue and it’s quite interesting, with different proposals. In short it seems you can try to use hooks for VM (http://docs.opennebula.org/5.0/integration/infrastructure_integration/hooks.html) but again I’d start with that post.
Cheers!