virttest.env_process: kill the tcpdump thread from postprocess

We should kill the tcpdump thread every time we are postprocessing
the test case to ensure that all log files will be closed properly.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
virtttest.utils_env: implement locking for tcpdump thread

Instead of killing and starting the tcpdump thread we will use locking
mechanism. It's a cleaner and better way to ensure that the
address_cache won't be changed during dumping the data.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>