xen/sched: modify cpupool_domain_cpumask() to be an unit mask

xen/sched: modify cpupool_domain_cpumask() to be an unit mask

cpupool_domain_cpumask() is used by scheduling to select cpus or to
iterate over cpus. In order to support scheduling units spanning
multiple cpus rename cpupool_domain_cpumask() to
cpupool_domain_master_cpumask() and let it return a cpumask with only
one bit set per scheduling resource.

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