Recently, more and more code was refactored to use an addr_family
integer to distinguish between IPv4 and IPv6.

Refactor nm_utils_get_ip_config_method() and nm_device_get_effective_ip_config_method()
to do that too. If we use different identifiers, we need to translate from one to
another and its inconsistent. Also, accessing a GType is an unnecessary function call,
instead of a plain constant.