Some unsupported storage controllers (e.g. floppy) report a nil
boot_priority which results in a failed sort. Select only supported
storage controllers to avoid that case.
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
