qdev: Fix qdev_try_create() for bus-less devices

Commit 7426aa72c36c908a7d0eae3e38568bb0a70de479 (nand: Don't inherit
from Sysbus) made NAND a device rather than SysBus device. This led to all
boards using the device (akita, borzoi, spitz, terrier, tosa, axis-dev88)
crashing on "info qtree".

The difference to the bus-less ARMCPU is that the device was still being
created by qdev_create() rather than object_new(), which makes assumptions
about a NULL BusState being the SysBus. Fix that.

A longer-term solution will be to stop using qdev_create().

Reported-by: Markus Armbruster <armbru@redhat.com>
Cc: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>