hw/acpi/piix4: Convert reset handler to DeviceReset

hw/acpi/piix4: Convert reset handler to DeviceReset

The PIIX4/PM is a PCI device within the PIIX4 chipset, it will be reset
when the PCI bus it stands on is reset.

Convert its reset handler into a proper Device reset method.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191010131527.32513-2-philmd@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>