rbd: Fix rbd diff for non-existent objects

Fixes: #6926
Signed-off-by: Adam Crume <adamcrume@gmail.com>
i do not make sure what do you mean "initial version" (version at the image creation?) , here is my understanding ,  at the second diff command, the image has been changed since we created snapshot ,  i think diff command should output the data that has been written to image after we created the snapshot.