Sort instance list in gnt-group list -opinst_list

The Python code currently sorts this, but the Haskell code not.

This should maybe have a test, but I'm not sure how far we want to
encode such properties in tests… (and the real reason I'm not adding
one is that we don't have a way to generate a random cluster _with_
instances).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>