fix bug about troubleshot image creation

"You cannot create a snapshot from an instance that has an
attached volume. Detach the volume, create the image,
and re-mount the volume."

In fact, there is no problem to create a snapshot from an
instance that has an attached volume.

Change-Id: I8b0dedbb0a3e09208824ca64e38179cea910d770
Closes-bug: #1318913