Neil, you can't revert back to a volume/disk snapshot. Look at volume snapshots as backups. You could restore from the back by creating a template from volume snapshot and then creating a VM from that template.

You have revert to snapshot functionality with VM snapshots. But you can't schedule a VM snapshot.

With the current releases of CCP, VM snapshots and Volume snapshots cannot co-exists, that is, you could only use one of the functionality with a VM/Disk.

This limitation is being addressed in the upcoming major ACS release and should be incorporated in CCP shortly after.