xen: sched: simplify ACPI S3 resume path.

xen: sched: simplify ACPI S3 resume path.

In fact, when domains are being unpaused:
 - it's not necessary to put the vcpu to sleep, as
   they are all already paused;
 - it is not necessary to call vcpu_migrate(), as
   the vcpus are still paused, and therefore won't
   wakeup anyway.

Basically, the only important thing is to call
pick_cpu, to let the scheduler run and figure out
what would be the best initial placement (i.e., the
value stored in v->processor), for the vcpus, as
they come back up, one after another.

Note that this is consistent with what was happening
before this change, as vcpu_migrate() calls pick_cpu.
But much simpler and quicker.

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