perf tools: fix event parsing of comma-separated tracepoint events

There are number of issues that prevent the use of multiple tracepoint events
being specified in a -e/--event switch, separated by commas.

For example, perf stat -e irq:irq_handler_entry,irq:irq_handler_exit ...  fails
because the tracepoint event parsing code doesn't recognize the comma separator
properly.

This patch corrects those issues.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Julia Lawall <julia@diku.dk>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reported-by: Michael Ellerman <michaele@au1.ibm.com>
LKML-Reference: <1291156021-17711-1-git-send-email-cjashfor@linux.vnet.ibm.com>
Signed-off-by: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>