The stub will be used on cases where sysbus.c is not compiled in (e.g.
*-user).

Note that code that uses NULL as the bus with qdev{_try,}_create()
implicitly uses sysbus_get_default() as the bus, and will still require
sysbus.c to be compiled in.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>