pv32pae: Probe for leaked SMEP/SMAP before switching to user mappings

pv32pae: Probe for leaked SMEP/SMAP before switching to user mappings

This allows XTF to panic() with a useful error message, rather than crashing
with an obscure error on the hypervisor console because of Xen failing to
create a bounce frame on the (now-user) stack.

Update the documentation, as Xen 4.7 no longer leaks SMEP/SMAP into guests.

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