hw/pci-host/i440fx: Add "i440fx" child property in board code

hw/pci-host/i440fx: Add "i440fx" child property in board code

The parent-child relation is usually established near a child's qdev_new(). For
i440fx this allows for reusing the machine parameter, thus avoiding
qdev_get_machine() which relies on a global variable.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230630073720.21297-9-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>