x86/svm: Fix a livelock when trying to run shadowed unpaged guests

x86/svm: Fix a livelock when trying to run shadowed unpaged guests

On AMD processors which support SMEP (Some Fam16h processors) and SMAP (Zen,
Fam17h), a guest which is running with shadow paging and clears CR0.PG while
keeping CR4.{SMEP,SMAP} set will livelock, as hardware raises #PF which the
shadow pagetable concludes shouldn't happen.

This occurs because hardware is running with host paging settings, which
causes the guests choice of SMEP/SMAP to actually take effect, even though
they shouldn't from the guests point of view.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>