hw/i386/acpi-build: Resolve redundant attribute

hw/i386/acpi-build: Resolve redundant attribute

The is_piix4 attribute is set once in one location and read once in
another. Doing both in one location allows for removing the attribute
altogether.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221026133110.91828-3-shentey@gmail.com>
Message-Id: <20221028103419.93398-3-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>