code: def _prepare_base_image(self):
          
          if not self.base_image:
              return defer.succeed(True)
  
          if self.cheap_copy:
              clone_cmd = "qemu-img"
              clone_args = "create -b %(base)s -f qcow2 %(image)s"
          else:
              clone_cmd = "cp"
              clone_args = "%(base)s %(image)s"
  
          clone_args = clone_args % {
              "base": self.base_image,
              "image": self.image,
          }
  
          log.msg("Cloning base image: %s %s'" % (clone_cmd, clone_args))
  
          d = utils.getProcessValue(clone_cmd, clone_args.split())
  
          def _log_result(res):
              log.msg("Cloning exit code was: %d" % res)
              return res
  
          def _log_error(err):
              log.err("Cloning failed: %s" % err)
              return err
  
          d.addCallbacks(_log_result, _log_error)
  
          return d
docs: I am a private method for creating (possibly cheap) copies of a
          base_image for start_instance to boot.