perf kvm stat live: Fix perf_evlist__add_pollfd error handling

perf kvm stat live: Fix perf_evlist__add_pollfd error handling

With the interface changed in following commit:

  2171a9256862 tools lib fd array: Allow associating an integer cookie with each entry

the perf_evlist__add_pollfd function now returns the fd position in the
pollfd array.

We need to change this function's error check condition.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Milian Wolff <mail@milianw.de>
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/r/1412179229-19466-3-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>