sched: always ask the scheduler to re-place the vcpu when the affinity changes

sched: always ask the scheduler to re-place the vcpu when the affinity changes

It's probably a good idea to re-evaluate placement whenever the
affinity changes.

This additionally has the benefit of removing scheduler-specific
exceptions introduced in git c/s e6a6fd63.

The conditionals surrounding vcpu_migrate() are left pending a re-work
of the logic to avoid the common case calling vcpu_migrate() twice (once
here, and once in context_saved().

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>