x86/hvm: Adjust hvm_nx_enabled() to match how Xen behaves

x86/hvm: Adjust hvm_nx_enabled() to match how Xen behaves

On Intel hardware, EFER is not fully switched between host and guest contexts.
In practice, this means that Xen's EFER.NX setting leaks into guest context,
and influences the behaviour of the hardware pagewalker.

When servicing a pagefault, Xen's model of guests behaviour should match
hardware's behaviour, to allow correct interpretation of the pagefault error
code, and to avoid creating observable difference in behaviour from the guests
point of view.

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