Use DeviceClass rather than SysBusDeviceClass in
nvram_sysbus_class_init().

Cc: pbonzini@redhat.com
Cc: marcandre.lureau@redhat.com
Signed-off-by: NMao Zhongyi <maozhongyi@cmss.chinamobile.com>
Signed-off-by: NZhang Shengju <zhangshengju@cmss.chinamobile.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181130093852.20739-15-maozhongyi@cmss.chinamobile.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>