xen: credit2: implement true SMT support

xen: credit2: implement true SMT support

In fact, right now, we recommend keepeing runqueues
arranged per-core, so that it is the inter-runqueue load
balancing code that automatically spreads the work in an
SMT friendly way. This means that any other runq
arrangement one may want to use falls short of SMT
scheduling optimizations.

This commit implements SMT awareness --similar to the
one we have in Credit1-- for any possible runq
arrangement. This turned out to be pretty easy to do,
as the logic can live entirely in runq_tickle()
(although, in order to avoid for_each_cpu loops in
that function, we use a new cpumask which indeed needs
to be updated in other places).

In addition to disentangling SMT awareness from load
balancing, this also allows us to support the
sched_smt_power_savings parametar in Credit2 as well.

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