Missing support for selecting signals for specific interfaces at dbus level

[pkcinna01]

When using python to access ofono, it is possible to listen to a specific signal and interface instance by specifying the signal name and path. dbus-next getInterface() only has a string for the interface name but it would be nice to also select the instance of the interface with a path.

Python example to listen for a specific modem:
systemBus.add_signal_receiver(someCallbackForSignalsToPhone, signal_name="CallAdded", bus_name="org.ofono", dbus_interface="org.ofono.VoiceCallManager", path="/hfp/org/bluez/hci0/dev_FC_D4_36_C0_2A_5A")