xen: sched: introduce 'adjust_affinity' hook.

xen: sched: introduce 'adjust_affinity' hook.

For now, just as a way to give a scheduler an "heads up",
about the fact that the affinity changed.

This enables some optimizations, such as pre-computing
and storing (e.g., in flags) facts like a vcpu being
exclusively pinned to a pcpu, or having or not a
soft affinity. I.e., conditions that, despite the fact
that they rarely change, are right now checked very
frequently, even in hot paths.

Note that, as we expect many scheduler specific
implementations of the adjust_affinity hook to do
something with the per-scheduler vCPU private data,
this commit moves the calls to sched_set_affinity()
after that is allocated (in sched_init_vcpu()).

Note also that this, in future, may turn out as a useful
mean for, e.g., having the schedulers vet, ack or nack
the changes themselves.

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