When a virtual machine reach the `DONE` state, the files under
`/var/lib/one/vms/<VMID>` and the log file `/var/lib/one/<VMID>.log`
are not removed for auditing purpose[1].

The `clean-done-vms` script will cleanup thoses files and directories
in two cases:

- for VM in state `DONE` for longer than a configurable time (30 days
  by default)
- for directories and files not associated with any known VMs in
  OpenNebula database (probably already purged during `onedb purge-done`)

Footnotes:
[1]  https://github.com/OpenNebula/one/issues/2408