Split piix4_device_hotplug() into hotplug/unplug callbacks
and register them as "hotplug-handler" interface implementation of
PIIX4_PM device.

Replace pci_bus_hotplug() wiring with setting link on
PCI BUS "hotplug-handler" property to PIIX4_PM device.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>