perf tools: Auto-complete for events with ':'

It's a follow up patch for a previous patch "perf tool: Return all
events as auto-completions after comma".

With this patch, auto-completion can work well for events with a ':'.
For example:

  root@skl:/tmp# perf stat -e block:block_<TAB>
  block:block_bio_backmerge   block:block_rq_complete
  block:block_bio_bounce      block:block_rq_insert
  block:block_bio_complete    block:block_rq_issue
  block:block_bio_frontmerge  block:block_rq_remap
  block:block_bio_queue       block:block_rq_requeue
  block:block_bio_remap       block:block_sleeprq
  block:block_dirty_buffer    block:block_split
  block:block_getrq           block:block_touch_buffer
  block:block_plug            block:block_unplug

  root@skl:/tmp# perf stat -e block:block_rq_<TAB>
  block:block_rq_complete  block:block_rq_issue     block:block_rq_requeue
  block:block_rq_insert    block:block_rq_remap

  root@skl:/tmp# perf stat -e block:block_rq_complete<TAB>
  block:block_rq_complete

  root@skl:/tmp# perf stat -e block:block_rq_complete

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1513973758-19109-1-git-send-email-yao.jin@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>