sysbus: Fix latent bug with onboard devices

The first call of sysbus_get_default() creates the main system bus and
stores it in QOM as "/machine/unattached/sysbus".  This must not
happen before main() creates "/machine", or else container_get() would
"helpfully" create it as "container" object, and the real creation of
"/machine" would later abort with "attempt to add duplicate property
'machine' to object (type 'container')".  Has been that way ever since
we wired up busses in QOM (commit f968fc68, v1.2.0).

I believe the bug is latent.  I got it to bite by trying to
qdev_create() a sysbus device from a machine's .instance_init()
method.

The fix is obvious: store the main system bus in QOM right after
creating "/machine".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190308131445.17502-5-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>