perf stat record: Write stat round events on record

Writing stat round events on 'perf stat record' for each interval round.
In non interval mode we store round event after the last stat event.

Committer note:

After the patch:

  $ perf report -D | grep PERF_RECORD | grep ROUND
  0x852 [0x18]: PERF_RECORD_STAT_ROUND
  $

Reported-by: Kan Liang <kan.liang@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1446734469-11352-9-git-send-email-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>