Previously, nm_bus_manager_register_object() would take various D-Bus
skeleton objects that were associated with one NMExportedObject.
This was confusing, in that these skeleton objects are all for the
same NMObject but correspond to different D-Bus interfaces.

Also, setting the D-Bus property "Managed" of a Device is broken
because we might retrieve the wrong skeleton.

Now, the NMBusManager has a reference to the exported object directly.
The skeleton interface instances instead are now exposed by the NMExportedObject.