vz: fix disk enumeration
    
    If we want to delete all disks for container or vm
    we should make a loop from 0 to NumberOfDisks and always
    use zero index in PrlVmCfg_GetHardDisk to get disk handle.
    When we delete first disk after that numbers of other disks
    will be changed, start from 0 to NumberOfDisks-1.
    That's why we should always use zero index.