netperf: stop tcpdump process during performance testing

This patch was wrongly dropped in original refactor.

Tcpdump brings extra latency, it causes the guest to batch
more bytes per packet. We should stop tcpdump to drop the
effect, it will be restarted at the beginning of next case.

Reported-by: Wenli Quan <wquan@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Acked-by: Wenli Quan <wquan@redhat.com>