xen: create xen_create/destroy_contiguous_region() stubs for PVHVM only builds

xen_create_contiguous_region()/xen_create_contiguous_region() are PV-only,
they both contain xen_feature(XENFEAT_auto_translated_physmap) check and
bail in the very beginning.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>