UrfArbitrator: make it possible to add devices to the arbitrator's list
 from outside of the arbitrator.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>
UrfDevice: clean up, correct minor errors in return values

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>
UrfOfonoManager: implement a "manager" to add oFono modems as radios to kill

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>
UrfDeviceOfono: implement a UrfDeviceOfono type for oFono modems.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>