perf tools: Default to cpu// for events v5

When an event fails to parse and it's not in a new style format,
try to parse it again as a cpu event.

This allows to use sysfs exported events directly without //, so you can use

perf record -e mem-loads ...

instead of

perf record -e cpu/mem-loads/

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1366480949-32292-1-git-send-email-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>