libxl supports the domU.cfg files from xend. Their syntax is equal anyway, but some aspects of the "old" disk=[] syntax needed an update to handle more use cases. In the end this translates to "existing domU.cfg files must be supported". This means that even the names of the devices (such as "hda") are accepted on the libxl side. I'm sure there are domU.cfg files with 'hd?' out there, and TW will simply breaks in those VMs. For no good reason.