trace: pass trace-events to tracetool as a positional param

Instead of reading the contents of 'trace-events' from stdin,
accept the filename as a positional parameter. This also
allows for reading from multiple files, though this facility
is not used at this time.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1475588159-30598-20-git-send-email-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>