libnm: extend NMSettInfoSetting with an offset to the private data

NMSetting instances either have no private data, they use
g_type_add_class_private(), or they embed the private data in the
NMSetting struct.

In all cases, we can find the private data at a fixed offset. Track that
offset in the NMSettInfoSetting meta data.

This will be useful, because properties really are stored in simple
fields, like a boolean property can be stored in a "bool" field. We will
extend the property meta data to track the offset of this property
field, but we also need to know where the offset starts.

src/libnm-core-impl/nm-setting-ovs-dpdk.c

@@ -148,5 +148,5 @@ nm_setting_ovs_dpdk_class_init(NMSettingOvsDpdkClass *klass)
 
     g_object_class_install_properties(object_class, _PROPERTY_ENUMS_LAST, obj_properties);
 
-    _nm_setting_class_commit(setting_class, NM_META_SETTING_TYPE_OVS_DPDK);
+    _nm_setting_class_commit(setting_class, NM_META_SETTING_TYPE_OVS_DPDK, NULL, NULL, 0);
 }