The existing constructor receives a perf_event_attr filled with the
event type and the config.

To reduce the boilerplate for tracepoints, provide a new constructor,
perf_evsel__newtp() that receives the tracepoint name and will open
the debugfs file, call into libtraceevent new pevent_parse_format file
to fill its ->tp_format member, so that users can then just call
perf_evsel__field() to access its fields.

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 <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/n/tip-6du8dl1hz0y5l4cybodye7hn@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>