clone_vm: refactor locking further

clone_vm: refactor locking further

introduce a new helper handling
- obtaining the flock
- (re)loading the config
- checking that the 'create' lock is still there

before calling a passed-in sub with the current config, since this
pattern was used quite a lot here.

intentionally changed behaviour:
- flock is now held for the post_clone hook call
- failure to remove the 'create' lock or to move the config to the
  target node if applicable will not undo the clone, since either is
  trivially fixable ('pct unlock' or a no-op migration), and copying all
  those volumes might have been quite expensive..

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>