perf python: Make twatch.py use soft dummy event, freq=0

To not sample, what we want are just the PERF_RECORD_ lifetime events
for threads, using the default, PERF_TYPE_HARDWARE +
PERF_COUNT_HW_CYCLES and freq=1 (the default), makes perf reenable
irq_vectors:local_timer_entry, disabling nohz, not good for some use
cases where all we want is to get notifications when threads comes and
goes...

Fix it by using PERF_TYPE_SOFTWARE (no counter rotation) and
PERF_COUNT_SW_DUMMY (created by Adrian so that we could have access to
those PERF_RECORD_ goodies).

Reported-by: Luiz Fernando Capitulino <lcapitulino@redhat.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jaroslav Skarvada <jskarvad@redhat.com>
Cc: Jeremy Eder <jeder@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-kfsijirfrs6xfhkcdxeoen06@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>