[libvirt] [PATCH rfc v4 0/6] storage:dir: ploop volumes support

[Olga Krishtal]

v4:
- fixed identation issues.
- in case of .uploadVol, DiskDescriptor.xml is restored.
- added check of ploops'accessibility

v3:
- no VIR_STORAGE_VOL_PLOOP type any more
- adapted all patches according to previous change
- fixed comments 

v2:
- fixed memory leak
- chenged the return value of all helper functions to 0/-1.
Now check for success is smth like that: vir****Ploop() < 0
- fixed some identation issues.