Fix type error in LUInstanceChangeGroup

If a specific list of groups has been requested, then the code used
that, without transforming it to a (frozen)set first, which results
in:

 unsupported operand type(s) for &: 'list' and 'frozenset'

Trivial fix is to do that in the 'then' branch.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>