app/testpmd: force user to stop forwarding when changing port/core list

app/testpmd: force user to stop forwarding when changing port/core list

Testpmd has the capability of changing the forwarding cores and ports in runtime.
If these are changed when forwarding, two issues may be encountered:

- If "show config fwd" is used, changes made in the core list are applied.
  Therefore, trying to stop forwarding may hang testpmd,
  since it could be waiting for cores to stop that are not actually running anything

- If the port list is changed, when stopping forwarding,
  it may miss the stats of some of the ports that were actually being used.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>