hw/s390x: Clean up global variable shadowing in quiesce_powerdown_req()

hw/s390x: Clean up global variable shadowing in quiesce_powerdown_req()

Fix:

  hw/s390x/sclpquiesce.c:90:22: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
      QuiesceNotifier *qn = container_of(n, QuiesceNotifier, notifier);
                       ^
  hw/s390x/sclpquiesce.c:86:3: note: previous declaration is here
  } qn;
    ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20231010115048.11856-7-philmd@linaro.org>