evtchn: implement EVTCHNOP_set_priority and add the set_priority hook

evtchn: implement EVTCHNOP_set_priority and add the set_priority hook

Implement EVTCHNOP_set_priority.  A new set_priority hook added to
struct evtchn_port_ops will do the ABI specific validation and setup.

If an ABI does not provide a set_priority hook (as is the case of the
2-level ABI), the sub-op will return -ENOSYS.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>