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 commit adds a new error message to be raised if a VM has no
supported storage controllers. This lets us differentiate between two
different "controller not found" scenarios:
1. If we are looking for a controller that we're expecting to find (i.e.
one that was recorded in the disk metadata file)
2. If we are poking around for the *best* controller to use in a
configuration task
