Drop an unreachable fallback bus assignment to SysBus.

If no ,bus= is specified, only search busses recursively for bus type if
the DeviceClass has a bus_type specified. Handle resulting NULL cases.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Message-id: 1366077021-28882-1-git-send-email-afaerber@suse.de
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>