KVM Test: Update for making netperf.py more stable

KVM Test: Update for making netperf.py more stable

1. Stop tcpdump by close()
Current we stop tcpdump by sending SIGSTOP to tcpdump process,
and do not do any clean up. e.g. release the lock.  Then send
SIGCOT to tcpdump process again. But if we fail continue tcpdump
process. Framework will think tcpdump still work and do not
start a new one in next case.
Then autotest could not refresh address_cache after guest's ip changed
So directly close() tcpdump, It will be started at the beginning of
next case.

2. kill netserver properly
netperf test doesn't handle all exceptions correctly, some exception
cause netserver isn't be killed, and the following netperf test will
fail. This patch will fix this problem.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Signed-off-by: Feng Yang <fyang@redhat.com>
git-svn-id: svn://test.kernel.org/autotest/trunk@5368 592f7852-d20e-0410-864c-8624ca9c26a4