Handle ofono restart

 Urfkill was crashing when ofono re-started because resources where not properly released when a ModemRemoved signal was received. Also, the id counter was not being reset and urf_device_ofono_get_modem_path was being called with a wrong argument.

The PR fixes

https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1346685