xen/sched: introduce struct sched_resource

xen/sched: introduce struct sched_resource

Add a scheduling abstraction layer between physical processors and the
schedulers by introducing a struct sched_resource. Each scheduler unit
running is active on such a scheduler resource. For the time being
there is one struct sched_resource per cpu, but in future there might
be one for each core or socket only.

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