Move all storage bus logic into the storage controller class. Since most
of the storage controller interaction only cares about the storage
controller name, we can simplify #get_controller and isolate the storage
controller detection-type logic in the StorageControllerArray.
This makes it easier to check if the required controller can be found,
and automatically raise an error if it is not.
Add a #storage_bus method to the StorageController class as a shorthand
way to check the general storage controller type.
