- Make sure devices are added to an interface if the interface is registered
  e.g. as a module.
- Make sure interface data descriptors are inserted into the interface's list
  by adding a kobject to them and calling kobject_register.
- Create interface_remove_data() for explicit removal of data descriptor.
- Break common pieces into separate functions.
- Add lots of comments.
- Add class_list to struct device, so we can access the devices registered
  with the class.