'Return an Image object for a disk with the given name.
:param instance: the instance which owns this disk
:param name: The name of the disk
:param image_type: (Optional) Image type.
Default is CONF.libvirt.images_type.
:return: An Image object for the disk with given name and instance.
:rtype: Image'

def by_name(self, instance, name, image_type=None):

    backend = self.backend(image_type)
    return backend(instance=instance, disk_name=name)
