VMX: disable SMEP feature when guest is in non-paging mode

VMX: disable SMEP feature when guest is in non-paging mode

SMEP is disabled if CPU is in non-paging mode in hardware.
However Xen always uses paging mode to emulate guest non-paging
mode with HAP. To emulate this behavior, SMEP needs to be manually
disabled when guest switches to non-paging mode.

We met an issue that, SMP Linux guest with recent kernel (enable
SMEP support, for example, 3.5.3) would crash with triple fault if
setting unrestricted_guest=0 in grub. This is because Xen uses an
identity mapping page table to emulate the non-paging mode, where
the page table is set with USER flag. If SMEP is still enabled in
this case, guest will meet unhandlable page fault and then crash.

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
Committed-by: Keir Fraser <keir@xen.org>