conf: snapshot: Avoid autogenerating duplicate snapshot names

conf: snapshot: Avoid autogenerating duplicate snapshot names

The snapshot name generator truncates the original file name after a '.'
and replaces the suffix with the snapshot name. If two disks source
images differ only in the suffix portion, the generated name will be
duplicate.

Since this is a corner case just error out stating that a duplicate name
was generated. The user can work around this situation by providing
the file names explicitly.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1283085