xen/sched: use new sched_unit instead of vcpu in scheduler interfaces

xen/sched: use new sched_unit instead of vcpu in scheduler interfaces

In order to prepare core- and socket-scheduling use a new struct
sched_unit instead of struct vcpu for interfaces of the different
schedulers.

Rename the per-scheduler functions insert_vcpu and remove_vcpu to
insert_unit and remove_unit to reflect the change of the parameter.
In the schedulers rename local functions switched to sched_unit, too.

Rename alloc_vdata and free_vdata functions to alloc_udata and
free_udata.

For now this new struct will contain a domain, a vcpu pointer and a
unit_id only and is allocated at vcpu creation time.

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