domctl: replace cpumask_weight() uses

domctl: replace cpumask_weight() uses

In one case it could easily be replaced by range checking the result of
a subsequent operation, and in general cpumask_next(), not always
needing to scan the whole bitmap, is more efficient than the specific
uses of cpumask_weight() here. (When running on big systems, operations
on CPU masks aren't cheap enough to use them carelessly.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>