add a domain_tot_pages() helper function

add a domain_tot_pages() helper function

This patch adds a new domain_tot_pages() inline helper function into
sched.h, which will be needed by a subsequent patch.

No functional change.

NOTE: While modifying the comment for 'tot_pages' in sched.h this patch
      makes some cosmetic fixes to surrounding comments.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: George Dunlap <George.Dunlap@eu.citrix.com>
Acked-by: Julien Grall <julien@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>