TODOs for vzplbackup

- More error handling and precaution
-- Detect if a container is actually using ploop as storage device
- Hook-Script to run prior creating the snapshot (automysqlbackup, etc)
- Allow exclusion of containers while running with --all switch
- Allow "vzctl compact" prior backup to reduce space used (good idea!?)
- Amend VE_ROOT and VE_PRIVATE in restored configuration to make sure
  the config matches with the directory it got restored to
- and more
