hw/acpi/piix4: change smm_enabled from int to bool

hw/acpi/piix4: change smm_enabled from int to bool

This is in preparation for conversion to a qdev property.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Ani Sinha <ani@anisinha.ca>
Message-Id: <20220528091934.15520-3-mark.cave-ayland@ilande.co.uk>
[PMD: Change simm_enabled from int to bool, suggested by Ani Sinha]
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>