Add FixedIP.floating_ips dynamic property

This adds a lazy-loading FixedIP.floating_ips property which mimics
the use of the backref between the two objects in the SQLAlchemy
model. This doesn't introduce a new database query because it was
being used as a lazy-load in network manager already.

Related to blueprint nova-network-objects

Change-Id: I2f13f7a0463486e51c2317e02c2e7b378ac9e55d

nova/objects/fixed_ip.py

@@ -42,6 +42,13 @@ class FixedIP(obj_base.NovaPersistentObject, obj_base.NovaObject):
         'network': fields.ObjectField('Network', nullable=True),
         }
 
+    @property
+    def floating_ips(self):
+        # NOTE(danms): avoid circular import
+        from nova.objects import floating_ip
+        return floating_ip.FloatingIPList.get_by_fixed_ip_id(self._context,
+                                                             self.id)
+
     @staticmethod
     def _from_db_object(context, fixedip, db_fixedip, expected_attrs=None):
         if expected_attrs is None: