Provide pcihp specific callback to check if bus is hotpluggable
and consolidate its scattered hotplug criteria there.
While at it clean up no longer needed
   qbus_set_hotplug_handler(BUS(bus), NULL)
workarounds since callback makes qbus_is_hotpluggable() return
correct answer even if hotplug_handler is set on bus.

PS:
see ("pci: fix 'hotplugglable' property behavior") for details
why callback was introduced.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20230302161543.286002-35-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>