core/empty_slot: Convert sysbus init function to realize function

core/empty_slot: Convert sysbus init function to realize function

Use DeviceClass rather than SysBusDeviceClass in
empty_slot_class_init().

Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181130093852.20739-5-maozhongyi@cmss.chinamobile.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>