Changing a disk's device type in the custom create form asked for its
buses at /archs/undefined/machines/undefined/...: the script read the
architecture and machine from a select[name=arch] and #machine-control
the page does not have. The request failed (500) and the bus list kept
the disk's buses, so a CD-ROM could be put on virtio. The script takes
both from the page's own URL.
