xen: credit2: make the cpu to runqueue map per-cpu

xen: credit2: make the cpu to runqueue map per-cpu

Instead of keeping an NR_CPUS big array of int-s,
directly inside csched2_private, use a per-cpu
variable.

That's especially beneficial (in terms of saved
memory) when there are more instance of Credit2 (in
different cpupools), and also helps fitting
csched2_private itself into CPU caches.

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