'Retrieve information about actual disk sizes of an instance.
:param instance: nova.objects.Instance
:param block_device_info:
Optional; Can be used to filter out devices which are
actually volumes.
:return:
json strings with below format::
"[{\'path\':\'disk\',
\'type\':\'raw\',
\'virt_disk_size\':\'10737418240\',
\'backing_file\':\'backing_file\',
\'disk_size\':\'83886080\'
\'over_committed_disk_size\':\'10737418240\'},'

def get_instance_disk_info(self, instance, block_device_info=None):

    raise NotImplementedError()
