xen/sched: let pick_cpu return a scheduler resource

xen/sched: let pick_cpu return a scheduler resource

Instead of returning a physical cpu number let pick_cpu() return a
scheduler resource instead. Rename pick_cpu() to pick_resource() to
reflect that change.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>