trace-events: Add hmp completion

Add completion for the trace event names in the hmp trace-event
command.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1439548063-18410-1-git-send-email-dgilbert@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>