xmlgen: Set `default_service` and `default_path` when known
Otherwise, inexistent freedesktop
ones are assumed.
E.g I generated code through zbus-xmlgen --session org.gnome.ScreenSaver /org/gnome/ScreenSaver
but generated proxy won't work as it doesn't specify the service and path:
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: MethodError("org.freedesktop.DBus.Error.UnknownMethod", Some("No such interface “org.gnome.ScreenSaver” on object at path /org/freedesktop/ScreenSaver"), Msg { type: Error, sender: ":1.98242", reply-serial: 2, body: Signature: [
s (115),
] })', src/main.rs:30:39