Don't call os.Exit explicitly to avoid lost of logs

[WeiZhang555]

Do not call os.Exit directly in codes, but call "return cli.NewExitError"
instead, so that after the program terminated, glog can flush and no
logs will be lost.

Signed-off-by: Zhang Wei zhangwei555@huawei.com

[WeiZhang555]

The reason that this commit works is that in https://github.com/hyperhq/runv/blob/master/main.go#L144-L148 , there is a glog.Flush() to flush all the logs after the APP exits, but if we call os.Exit in middle directly, there's no way to flush all the logs, hence cached logs would be lost.
With this modification, glog.Flush will always be called when program exits normally (except panic), and we could get all logs for debugging.