fix(backups/MixinBackupWriter): clean VM dir after backup
fix(backups/MixinBackupWriter): clean VM dir after backup

Otherwise, it might trigger a chain reaction which will force all VDIs to be fully exported:

1. a single VDI chain is corrupted
2. it gets removed
3. the linked backups are removed
4. all other VDIs are now unused and are removed as well
5. all VDIs must now be fully exported

lmcro/xo-web,vatesfr/xo-web,vatesfr/xo-web,lmcro/xo-web,lmcro/xo-web