usb-host: use qdev for -usbdevice + rework.

usb-host: use qdev for -usbdevice + rework.

Changes:

 * We don't create/delete devices, we attach/detach them instead.
 * The separate autofilter list is gone, we simply walk the list
   of devices directly instead.
 * Autofiltering is done unconditionally now.  Non-auto device scan
   code got dropped.
 * Autofiltering turns off the timer if there is nothing to do, it
   runs only in case there are unattached host devices.

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