snapshot: Saner use of uuid
    
    Most of the code base is fairly consistent about using the name
    'uuidstr' when dealing with a formatted human-readable form, and
    'uuid' when dealing with the smaller raw bytes form. Fix
    snapshot_conf to comply, as well as reducing the scope of a human
    string to only the error message that needs it.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>