objects: Makes sure Instance._save methods are called

This patch makes sure that _save hooks are called for Instance object's
related objects even when they are set to None (if they are at all
nullable)

We will need this for numa_topology, as we need to be able to update it
on save, and make sure it's gone if the new one is None.

(cherry picked from commit 7335af1ea36be5ecf3f55b8c23857f678bbcbe39)

Change-Id: Ied9e760b40446661a6269e41df6091aa16fbfc82
Merge "objects: Makes sure Instance._save methods are called" into stable/juno