hw/acpi/piix4: convert smm_enabled bool to qdev property

hw/acpi/piix4: convert smm_enabled bool to qdev property

This allows the smm_enabled value to be set using a standard qdev property instead
of being referenced directly in piix4_pm_init().

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: <20220528091934.15520-4-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>