x86: frame table related improvements

x86: frame table related improvements

- fix super page frame table setup for memory hotplug case (should
  create full table, or else the hotplug code would need to do the
  necessary table population)
- simplify super page frame table setup (can re-use frame table setup
  code)
- slightly streamline frame table setup code
- fix (tighten) a BUG_ON() and an ASSERT() condition
- fix spage <-> pdx conversion macros (they had no users so far, and
  hence no-one noticed how broken they were)

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