constructor functions are ugly, because code is running before
main() starts. Instead, as the registration code for NMSetting types
is insid the GType constructor, we just need to ensure at the
right place, that the GType was created.
The right place here is _register_settings_ensure_inited(), because
that is called before we need the registration information.
Normalizing can be complicated, as settings depend on each other and possibly
conflict.
That is, because verify() must exactly anticipate whether normalization will
succeed and how the result will look like. That is because we only want to
modify the connection, if we are sure that the result will verify.
Hence, verify() and normalize() are strongly related. The implementation
should not be spread out between NMSettingOvsInterface:verify(),
NMSettingOvsPatch:verify() and _normalize_ovs_interface_type().
Also, add some unit-tests.
