xen/sched: support allocating multiple vcpus into one sched unit

xen/sched: support allocating multiple vcpus into one sched unit

With a scheduling granularity greater than 1 multiple vcpus share the
same struct sched_unit. Support that.

Setting the initial processor must be done carefully: we can't use
sched_set_res() as that relies on for_each_sched_unit_vcpu() which in
turn needs the vcpu already as a member of the domain's vcpu linked
list, which isn't the case.

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