adb: QOM'ify Apple Desktop Bus

It was not a qbus before, turn it into a first-class bus and initialize
it properly from CUDA. Leave it a global variable as long as devices are
not QOM'ified yet.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
