xen: sched: harmonize debug dump output among schedulers.

xen: sched: harmonize debug dump output among schedulers.

Information we currently print for idle vCPUs is
rather useless. Credit2 already stopped showing that,
do the same for Credit and RTDS.

Also, define a new CPU status dump hook, which is
not defined by those schedulers which already dump
such info in other ways (e.g., Credit2, which does
that while dumping runqueue information).

This also means that, still in Credit2, we can keep
the runqueue and pCPU info closer together.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Meng Xu <mengxu@cis.upenn.edu>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>