After talking to Andrew, I realized I was off-base here.

 >> This looks a bit suspicious.  Should we be also comparing the object
>> pointer, not just the instance pointer here?
>
> Assuming that only one object can have the instance it shouldn't
> matter.  When the object or the instance are removed, the property
> change record is also removed so it won't accidentally point at a
> different instance.

I for some reason was thinking we're tracking l_dbus_interface objects, 
not interface_instance.  So after being being reminded of that, the code 
is just fine.

Regards,
-Denis