x86/xen: Set default memory type for PV guests to WB

When running as an unprivileged PV guest under Xen (not dom0), the
default MTRR memory type should be write-back.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20230615123959.12298-1-jgross@suse.com