x86: restore reserving of IO-APIC pages in XENMEM_machine_memory_map output

x86: restore reserving of IO-APIC pages in XENMEM_machine_memory_map output

Commit d1222afda4 ("x86: allow Dom0 read-only access to IO-APICs") had
an unintended side effect: By no longer adding IO-APIC pages to Dom0's
iomem_caps these also no longer get reported as reserved in the machine
memory map presented to it (which got added there intentionally by
commit b8a456caed ["x86: improve reporting through
XENMEM_machine_memory_map"] because many BIOSes fail to add these).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>