xen: sched: generalize scheduling related perfcounter macros

xen: sched: generalize scheduling related perfcounter macros

Moving some of them from sched_credit.c to generic scheduler code.
This also allows the other schedulers to use perf counters equally
easy.

This change is mainly preparatory work for what stated above. In fact,
it mostly does s/CSCHED_STAT/SCHED_STAT/, and, in general, it implies
no functional changes.

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>