device-manager: Provide a method for prefering/defering a device.
This allows clients to edit the priroity order. What is not yet in place is the initialisation of that priority list
when new devices are detected or the cleaning (remove holes) when devices are removed.
In order to keep the storage transparent I will likely remove the write functionality and replace it with a
simple rename method.
I also still need to expose the priority itself when reading the data.