xen/cpupool: add missing bits for per-cpupool scheduling granularity

xen/cpupool: add missing bits for per-cpupool scheduling granularity

Even with storing the scheduling granularity in struct cpupool there
are still a few bits missing for being able to have cpupools with
different granularity (apart from the missing interface for setting
the individual granularities): the number of cpus in a scheduling
unit is always taken from the global sched_granularity variable.

So store the value in struct cpupool and use that instead of
sched_granularity.

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