perf tools: Add event_update event unit type

Adding unit type 'event update' event, that stores/transfer events unit
name. The unit name is part of the perf stat output data.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1445784728-21732-22-git-send-email-jolsa@kernel.org


[ Rename __alloc() to __new() for consistency ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>