trace: Provide a generic tracing event descriptor

Uses tracetool to generate a backend-independent tracing event description
(struct TraceEvent).

The values for such structure are generated with the non-public "events"
backend ("events-c" frontend).

The generation of the defines to check if an event is statically enabled is also
moved to the "events" backend ("events-h" frontend).

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
