The nr_events in trace__run was local, but we will need it in other
trace methods, move it to struct trace.

We'll also need the number of events per thread, so introduce a
nr_events method for that in struct thread_trace.

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>
Link: http://lkml.kernel.org/n/tip-ksutaz0mtejnf7e6az3ca1td@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>