tests.netperf: Update iptables stop command to ignore fail status

In some host 'service iptables stop' command exit with error code 5.
It will fail our case.

Signed-off-by: Feng Yang <fyang@redhat.com>
shared.cfg.guest-os.Windows: Ignore post_cmd status in multi_disk case

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #577 from FengYang/fix

tests.netperf: Update iptables stop command to ignore fail status.