xen: sched: provide some scratch space for not putting cpumasks on stack

xen: sched: provide some scratch space for not putting cpumasks on stack

directly, from schedule.c, for any scheduler that needs
it to use it.

In fact, Credit1 and RTDS needs this already. Credit2 is
also going to need it, for supporting hard affinity
(which is, typically, what requires a lot of cpumask
manipulations, inside various functions).

Therefore, let's define the scratch space at a broader
scope, to limit code duplication in handling it.

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