Hook into DeviceInfo->exit().

handle_destroy() must not free the state struct, this is handled
by the new usb_qdev_exit() function now.

qdev_free(usb_device) works now.

Fix usb hub to qdev_free() all connected devices on unplug.
Unplugging a usb hub works now.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>