'obj_from_primitive' of embedded objects is not used

Consider the following objects:

 @obj_base.VersionedObjectRegistry.register
 class Bar(obj_base.VersionedObject):
     fields = {
         'id': fields.IntegerField(),
         'name': fields.StringField(),
     }

     @classmethod
     def obj_from_primitive(cls, primitive, context=None):
         # do something...
         return super().obj_from_primitive(primitive, context)

 @obj_base.VersionedObjectRegistry.register
 class Foo(obj_base.VersionedObject):
     fields = {
         'name': fields.StringField(),
         'bar': fields.ObjectField('Bar'),
     }

If you deserialize and reserialize an instance of 'Foo', you'll note the 'obj_from_primitive' method from the 'Bar' object is never called. Things work correctly when working with 'Bar' directly. This appears to be because we are always calling the parents 'obj_from_primitive' function when deserializing child objects [1].

[1] https://github.com/openstack/oslo.versionedobjects/blob/2.0.2/oslo_versionedobjects/fields.py#L823