def get_backing_file(self, instance_id):
        cmd = ("qemu-img info {image_path} --output json".format(
            image_path=instance_image_path(instance_id)))

        try:
            image_info = json.loads(self.runner.run(cmd))
            return image_info['backing-filename']
        except (ValueError, TypeError) as e:
            LOG.error("Invalid value received from qemu: %s!", e)
        except __HOLE__:
            LOG.warning("Instance '%s' does not have backing file associated!",
                        instance_id)
dataset/ETHPy150Open MirantisWorkloadMobility/CloudFerry/cloudferry/lib/os/compute/libvirt.py/Libvirt.get_backing_file