xen/sched: add is_running indicator to struct sched_unit

xen/sched: add is_running indicator to struct sched_unit

Add an is_running indicator to struct sched_unit which will be set
whenever the unit is being scheduled. Switch scheduler code to use
unit->is_running instead of vcpu->is_running for scheduling decisions.

At the same time introduce a state_entry_time field in struct
sched_unit being updated whenever the is_running indicator is changed.
Use that new field in the schedulers instead of the similar vcpu field.

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