perf: Introduce a new "round of buffers read" pseudo event

In order to provide a more rubust and deterministic reordering
algorithm, we need to know when we reach a point where we just
did a pass through over every counter buffers to read every thing
they had.

This patch introduces a new PERF_RECORD_FINISHED_ROUND pseudo event
that only consist in an event header and doesn't need to contain
anything.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Masami Hiramatsu <mhiramat@redhat.com>