qdev: Use NULL instead of local_err for qbus_child unrealize

Forcefully unrealize all children regardless of errors in earlier
iterations (if any). We should keep going with cleanup operation
rather than report an error immediately. Therefore store the first
child unrealization failure and propagate it at the end. We also
forcefully unregister vmsd and unrealize actual object, too.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
qdev: Add cleanup logic in device_set_realized() to avoid resource leak

At present, this function doesn't have partial cleanup implemented,
which will cause resource leaks in some scenarios.

Example:

1. Assume that "dc->realize(dev, &local_err)" executes successful
   and local_err == NULL;
2. device hotplug in hotplug_handler_plug() executes but fails
   (it is prone to occur). Then local_err != NULL;
3. error_propagate(errp, local_err) and return. But the resources
   which have been allocated in dc->realize() will be leaked.
Simple backtrace:
  dc->realize()
   |->device_realize
            |->pci_qdev_init()
                |->do_pci_register_device()
                |->etc.

Add fuller cleanup logic which assures that function can
goto appropriate error label as local_err population is
detected at each relevant point.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>