hw/qdev: Cosmetic around documentation

Add empty lines to have a clearer distinction between different
functions declarations.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Yanan Wang <wangyanan55@huawei.com>
Message-Id: <20211218130437.1516929-2-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>