split the old SysBus init function into an instance_init
and a Device realize function

Signed-off-by: xiaoqiang zhao <zxq_yx_007@163.com>
Message-id: 1465815255-21776-13-git-send-email-zxq_yx_007@163.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>