code: def backing_file(self):
          
  
          with open(self._path, 'rb') as f:
              f.seek(self.backing_file_offset)
              content = f.read(self.backing_file_size)
  
          path = content.decode()
          if len(path) == 0:
              return None
          return path
docs: When using linked clone this will return the path to the base image
  
          :returns: None if it's not a linked clone, the path otherwise