xen/sched: move struct task_slice into struct sched_unit

xen/sched: move struct task_slice into struct sched_unit

In order to prepare for multiple vcpus per schedule unit move struct
task_slice in schedule() from the local stack into struct sched_unit
of the currently running unit. To make access easier for the single
schedulers add the pointer of the currently running unit as a parameter
of do_schedule().

While at it switch the tasklet_work_scheduled parameter of
do_schedule() from bool_t to bool.

As struct task_slice is only ever modified with the local schedule
lock held it is safe to directly set the different units in struct
sched_unit instead of using an on-stack copy for returning the data.

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