Returns the size of the used space on the specified disk, in bytes.
PRL_RESULT PrlStatDisk_GetUsageDiskSpace( PRL_HANDLE hDiskStat, PRL_UINT64_PTR pnSize );
PrlApiStat.h
PRL_RESULT. Possible values:
PRL_ERR_INVALID_ARG - invalid handle or null pointer was passed.
PRL_ERR_SUCCESS - function completed successfully.