No longer rely on nm_connection_get_path() being meaningful in server.
It also was wrong. During update, nm_settings_connection_update()
would call
  nm_utils_log_connection_diff (replace_connection, NM_CONNECTION (self), ...
where replace_connection has no path set, and nothing was logged.

Fix it, by explicitly passing the D-Bus path. Also, because
nm-core-utils.c should be independent of nm-dbus-object.h.