Check for persistent disk before copying

The recent version of libcloudforensics don't have any nonce in the title so it makes collisions more likely if there are already copies of the disk that have been made. We should check for the existence of the disk before trying to copy it and change the output path, or potentially ask the user if they want to process the new disk (could potentially just give a command line example of processing the new disk). Maybe some of this could be upstreamed into libcloudforensics as well.

Locate the disk-copy handling and its libcloudforensics integration, then inspect how the destination path is currently chosen. Confirm how an existing persistent disk copy should be detected and whether the workflow should choose a new path or explain how to process the new disk. Done means collisions are handled safely and the chosen behavior is covered by the relevant tests or command-line flow.

Written by the indexing model from the issue text.