SCSI controllers are a lot like SATA controllers. This commit also
changes some controller detection logic to take boot priority into
account when selecting an appropriate controller.
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
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.
