x86/pv: Fill all Xen slots in init_guest_l4_table()

x86/pv: Fill all Xen slots in init_guest_l4_table()

There is a bug when using highmem-start= where some L4 directmap slots are not
audited in alloc_l4_table(), and not overwritten by init_guest_l4_table().

As highmem_start is only available in debug builds of the hypervisor, this
does not constitute a security issue.

Ensure that init_guest_l4_table() writes to all of the Xen slots.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>