vmx: Do not disable real EFER.NXE even when disabled by guest.

vmx: Do not disable real EFER.NXE even when disabled by guest.

We must not disable EFER.NXE in host mode since shadow code relies on
accessing shadow mappings with NX set.

We do not want to write EFER on every vmentry/vmexit if we can avoid
it, since it will be somewhat slow.

Finally, we don't believe that any guest relies on NX really being
disabled when EFER.NXE is cleared.

This given, it makes sense to ignore the guest's setting of EFER.NXE.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>