Comment 5 for bug 1570748

It looks like the problem is here:

https://github.com/openstack/nova/blob/9989a75384aeb6c6b398e7caf8dee140c6bf0423/nova/compute/utils.py#L462

When doing a revert_resize from the compute API, it calls nova.compute.utils.reverse_upsize_quota_delta which tries to lookup the old flavor using the flavor ID from the migration record rather than the instance, and since the flavor was deleted we get a 404.