qdev: Let machine hotplug handler to override bus hotplug handler

it will allow to return another hotplug handler than the default
one for a specific bus based device type. Which is needed to handle
non trivial plug/unplug sequences that need the access to resources
configured outside of bus where device is attached.

That will allow for returned hotplug handler to orchestrate wiring
in arbitrary order, by chaining other hotplug handlers when
it's needed.

PS:
It could be used for hybrid virtio-mem and virtio-pmem devices
where it will return machine as hotplug handler which will do
necessary wiring at machine level and then pass control down
the chain to bus specific hotplug handler.

Example of top level hotplug handler override and custom plug sequence:

  some_machine_get_hotplug_handler(machine){
      if (object_dynamic_cast(OBJECT(dev), TYPE_SOME_BUS_DEVICE)) {
          return HOTPLUG_HANDLER(machine);
      }
      return NULL;
  }

  some_machine_device_plug(hotplug_dev, dev) {
      if (object_dynamic_cast(OBJECT(dev), TYPE_SOME_BUS_DEVICE)) {
          /* do machine specific initialization */
          some_machine_init_special_device(dev)

          /* pass control to bus specific handler */
          hotplug_handler_plug(dev->parent_bus->hotplug_handler, dev)
      }
  }

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190228122849.4296-3-david@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

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