xen: tracing: introduce per-scheduler trace event IDs

xen: tracing: introduce per-scheduler trace event IDs

So that it becomes possible to create scheduler specific trace
records, within each scheduler, without worrying about the
overlapping, and also without giving up being able to recognise them
univocally. The latter is deemed as useful, since we can have more
than one scheduler running at the same time, thanks to cpupools.

The event ID is 12 bits, and this change uses the upper 3 of them for
the 'scheduler ID'. This means we're limited to 8 schedulers and to
512 scheduler specific tracing events. Both seem reasonable
limitations as of now.

This also converts the existing credit2 tracing (the only scheduler
generating tracing events up to now) to the new system.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>