xen: sched/credit: convert scheduling parameter to s_time_t when set

xen: sched/credit: convert scheduling parameter to s_time_t when set

Basically, instead of converting integers to s_time_t
at usage time (hot paths), do the convertion when the
values are set (cold paths).

This applies to the timeslice and the ratelimit
parameters of Credit1.

Note that, when changing the type of the fields of
struct csched_private (from unsigned to s_time_t),
ncpus is moved up a bit, for better packing.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>