xen/sched: Change vcpu_migrate_*() to operate on schedule unit

xen/sched: Change vcpu_migrate_*() to operate on schedule unit

vcpu_migrate_start() and vcpu_migrate_finish() are used only to ensure
a vcpu is running on a suitable processor, so they can be switched to
operate on schedule units instead of vcpus.

While doing that rename them accordingly.

Call vcpu_sync_execstate() for each vcpu of the unit when changing
processors in order to make that an explicit action (otherwise this
would happen later when either the vcpu is scheduled on the new
processor or another non-idle vcpu is scheduled on the old processor).

vcpu_move_locked() is switched to schedule unit, too.

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