x86: introduce create_perdomain_mapping()

x86: introduce create_perdomain_mapping()

... as well as free_perdomain_mappings(), and use them to carry out the
existing per-domain mapping setup/teardown. This at once makes the
setup of the first sub-range PV domain specific (with idle domains also
excluded), as the GDT/LDT mapping area is needed only for those.

Also fix an improperly scaled BUILD_BUG_ON() expression in
mapcache_domain_init().

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