x86/xen: create stubs for HVM-only builds in page.h

__pfn_to_mfn() is only used from PV code (mmu_pv.c, p2m.c) and from
page.h where all functions calling it check for
xen_feature(XENFEAT_auto_translated_physmap) first so we can replace
it with any stub to make build happy.

set_foreign_p2m_mapping()/clear_foreign_p2m_mapping() are used from
grant-table.c but only if !xen_feature(XENFEAT_auto_translated_physmap).

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