Also don't abuse the disk driver name to specify the SCSI controller
model anymore:

  <driver name='buslogic'/>

Use the newly added model attribute of the controller element for this:

  <controller type='scsi' index='0' model='buslogic'/>

The disk driver name approach is deprecated now, but still works for
backward compatibility reasons.

Update the documentation and tests accordingly.

Fix usage of the words controller and id in the VMX handling code. Use
controller, bus and unit properly.