trivial: comment about vif object address field

I always have to check if the VirtualInterface.address
field is an IP or a MAC address, so add a comment about
what it is.

Change-Id: I2fd31f2bcc00a0bba15b625c2d97f7d50e40e929

nova/objects/virtual_interface.py

@@ -33,6 +33,7 @@ class VirtualInterface(base.NovaPersistentObject, base.NovaObject):
 
     fields = {
         'id': fields.IntegerField(),
+        # This is a MAC address.
         'address': fields.StringField(nullable=True),
         'network_id': fields.IntegerField(),
         'instance_uuid': fields.UUIDField(),