rbd: make the code more readable

Make it a bit clearer and more readable.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1476519973-6436-1-git-send-email-lixiubo@cmss.chinamobile.com
CC: John Snow <jsnow@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>