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

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.