perf evlist: Add perf_evlist__id2sid method to get event ID related data

This will be helpful for PERF_FORMAT_GROUP samples where we need to
store ID related period value for each event.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-twmlgsbyim97p7cyohjwb1df@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>