hw/isa/piix3: Create power management controller in host device

hw/isa/piix3: Create power management controller in host device

The power management controller is an integral part of PIIX3 (function 3). So
create it as part of the south bridge.

Note that the ACPI function is optional in QEMU. This is why it gets
object_initialize_child()'ed in realize rather than in instance_init.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20231007123843.127151-14-shentey@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>