perf evlist: Fix id pos in perf_evlist__open()

Ensure the id_pos is correct when perf_evlist__open() is used.

This fixes a problem introduced in 75562573

 that broke 'perf kvm stat
live' in that this tool wasn't updated to use the sample_type bits
setting helpers.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: David Ahern <dsahern@gmail.com>
Acked-by: David Ahern <dsahern@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1378496412-2424-2-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>