Sidenote: instead of STALLing because the driver didn't configure the
endpoints properly, regular transfers now work. Thus, QEMU was able to
recognize that it received a Bulk-Only Transport CBW. Yay!

Additionally, to make life easier when debugging, usbctl was added; now
it can get endpoint statuses which were previously non-accessible from
shell scripts, after the transition to the new ctl+data endpoint
interface.