sched: improve domain creation tracing

sched: improve domain creation tracing

by doing the following two things:

 - move TRC_SCHED_DOM_{ADD,REM}, into the functions
   that do the actual scheduling-related domain
   initialization;

 - add two 'generic' DOM_{ADD,REM} events. They're
   made part of the TRC_DOM0 tracing class, as Dom0
   is, usually, from where domains are created and
   destroyed.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>