xen: sched: factor affinity helpers out of sched_credit.c

xen: sched: factor affinity helpers out of sched_credit.c

In fact, we want to be able to use them from any scheduler.

While there, make the moved code use 'v' for struct_vcpu*
variable, like it should be done everywhere.

No functional change intended.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Signed-off-by: Justin T. Weaver <jtweaver@hawaii.edu>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>