XSA-296: Don't rely on PV-L1TF side effects

XSA-296: Don't rely on PV-L1TF side effects

When CONFIG_SHADOW_PAGING is compiled out, PV-L1TF defaults to domain_crash()
on vulnerable hardware.  While this has technically demonstrated that Xen
isn't vulnerable to XSA-296, it's an unclean exit.

The pv64 case can already spot and correct the race condition, while the
pv32pae test is no different to running on non-L1TF vulnerable hardware
already.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>