x86/paging: use clear_guest() for zero-filling guest buffers

x86/paging: use clear_guest() for zero-filling guest buffers

While static arrays of all zeros may be tolerable (but are simply
inefficient now that we have the necessary infrastructure), using on-
stack arrays for this purpose (particularly when their size doesn't
have an upper limit enforced) is calling for eventual problems (even
if the code can be reached via administrative interfaces only).

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