sysbus: make SysBusDeviceClass::init optional

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: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 1363358063-23973-2-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>