Because missing optional storage source is not error. The patch
address only local files. Fixing other cases is a bit ugly.
Below is example of error notice in log now:

error: virStorageFileReportBrokenChain:427 :
   Cannot access storage file '/path/to/missing/optional/disk':
  	No such file or directory
Signed-off-by: NNikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>