pci/shpc: convert SHPC hotplug to use hotplug-handler API

pci/shpc: convert SHPC hotplug to use hotplug-handler API

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

Replace pci_bus_hotplug() wiring with setting link on PCI BUS
"hotplug-handler" property to PCI_BRIDGE_DEV device.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>