hw/acpi/piix4: move xen_enabled() logic from piix4_pm_init() to piix4_pm_realize()

hw/acpi/piix4: move xen_enabled() logic from piix4_pm_init() to piix4_pm_realize()

This logic can be included as part of piix4_pm_realize() and does not need to
be handled externally.

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: <20220528091934.15520-2-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>