qdev/scsi+usb: convert usb-storage to qdev.

qdev/scsi+usb: convert usb-storage to qdev.

Full coverage with properties and everything.  You can add virtual usb
sticks this way now:

  -drive if=none,id=pendrive,path=/some/where
  -device usb-storage,drive=pendrive

-usbdevice disk:/path/to/image continues to work.

Other side effects:
usb storage is listed in 'info block' now.
kvm tree should be able to boot from usb via extboot (untested though).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>