perf stat report: Process stat config event

Adding processing of stat config event and initialize stat_config
object.

Reported-by: Kan Liang <kan.liang@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
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/1446734469-11352-14-git-send-email-jolsa@kernel.org


[ Renamed 'stat' parameter to 'st' to fix 'already defined' build error with older distros (e.g. RHEL6.7) ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>