hw/s390x: Attach default virtio-net devices to the /machine/virtual-css-bridge

hw/s390x: Attach default virtio-net devices to the /machine/virtual-css-bridge

The initial virtio-net-ccw devices currently do not have a proper parent
in the QOM tree, so they show up under /machine/unattached - which is
somewhat ugly. Let's attach them to /machine/virtual-css-bridge/virtual-css
instead.

Message-ID: <20240701200108.154271-1-thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>