VMX: use a single, global APIC access page

VMX: use a single, global APIC access page

The address of this page is used by the CPU only to recognize when to
access the virtual APIC page instead. No accesses would ever go to this
page. It only needs to be present in the (CPU) page tables so that
address translation will produce its address as result for respective
accesses.

By making this page global, we also eliminate the need to refcount it,
or to assign it to any domain in the first place.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Tim Deegan <tim@xen.org>