trace: move util/buffer.c trace points into correct file

trace: move util/buffer.c trace points into correct file

The trace points for util/buffer.c were mistakenly put
in the io/trace-events file, instead of util/trace-events
in

  commit 892bd32ea38bbe9709ff0b6db3053bdf06eec9fb
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Thu Jun 16 09:39:50 2016 +0100

    trace: split out trace events for io/ directory

    Move all trace-events for files in the io/ directory to

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1473872624-23285-2-git-send-email-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>