The previous syntax was severely limited in its options
-usbdevice disk:/home/berrange/output.img
The new syntax is the same as for other disk types
-drive file=/home/berrange/output.img,if=none,id=usb-1,index=1
-device usb-storage,drive=usb-1
Again, the index= arg is wrong here, and will be removed in a
later merge
