[pve-devel] pve-container : make create_rootfs sub generic v2

[Alexandre Derumier]

This is a rework of my today patch serie

- I have added speficic sub for attach|dettach loops.
- I now use PVE::Storage::activate_volumes($vollist) in hooks
- I don't return loop:/ path anymore in volid_path sub.
  It's only use in 1 place, for update_config. 
  All other places (backup,create,...) need the real path.
  Also, this avoid the need to use $ms, which is related to mountpoint and not volid.

- create_roofs is generic, next step will to add support to allocate any mounpoint disk