Redirect output for gnt-* list/list-fields commands

This enabled the redirection; QA output after this patch should become
much much smaller (on a 'quick' QA, we are left with ~0.5MB output on
stdout and 64MB redirected output).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>