Just tried updating our pool and after updating the master had failed migration of VM (Ubuntu) and then was unable to get VM to start (Missing VDI error). Only solution was to roll master back to 7.4 and everything working again.
I ran into a similar situation when I upgraded to 7.5 this week. Did you check to see if there was a non-existent ISO mounted in the CD rom for the VM in question?
Thank you. That was my problem. I gave cd empty. Everything works.