perf tools: Only save the event formats we need

While opening a trace event counter, every events are saved in
the trace.info file. But we only want to save the
specifications of the events we are using.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <1251421798-9101-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>