sched: fix get_cpu_idle_time() with core scheduling

sched: fix get_cpu_idle_time() with core scheduling

get_cpu_idle_time() is calling vcpu_runstate_get() for an idle vcpu.
With core scheduling active this is fragile, as idle vcpus are assigned
to other scheduling units temporarily, and that assignment is changed
in some cases without holding the scheduling lock, and
vcpu_runstate_get() is using v->sched_unit as parameter for
unit_schedule_[un]lock_irq(), resulting in an ASSERT() triggering in
unlock in case v->sched_unit has changed meanwhile.

Fix that by using a local unit variable holding the correct unit.

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