sysbus: make SysBusDeviceClass::init optional

Make the SysBusDeviceClass::init optional, for devices which
genuinely don't need to do anything here. In particular, simple
devices which can do all their initialization in their
instance_init method don't need either a DeviceClass::realize
or SysBusDeviceClass::init method.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Acked-by: NAndreas Färber <afaerber@suse.de>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 1363358063-23973-2-git-send-email-peter.maydell@linaro.org
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/sysbus.c

@@ -137,6 +137,9 @@ static int sysbus_device_init(DeviceState *dev)
     SysBusDevice *sd = SYS_BUS_DEVICE(dev);
     SysBusDeviceClass *sbc = SYS_BUS_DEVICE_GET_CLASS(sd);
 
+    if (!sbc->init) {
+        return 0;
+    }
     return sbc->init(sd);
 }