We can change the property's D-Bus signature (and thus API) here
because querying the IP6Config object's properties caused NM to
crash. Apparently we forgot to change the type of the Address
property when we C&P-ed the IP4Config into the IP6Config, and
DBUS_TYPE_G_ARRAY_OF_ARRAY_OF_UINT is certainly the wrong type
to use since the backing object that dbus-glib would marshal
into the ARRAY_OF_ARRAY_OF_UINT wasn't that type, causing a
crash in dbus-glib when a client got the IP6Config.
