Add the following functions:

  perf_evlist__open()
  perf_evlist__close()

It's a simplified version of perf's evlist__open() without the sampling
id index calculations. We can try to merge it in the future when we need
it in some new libperf user.

Also adopt some helper evlist traversing macros. In the future we can
remove them from util/evlist.h, but that requires also some other
changes.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190721112506.12306-69-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>