x86/hvm: add SMEP support to HVM guest

x86/hvm: add SMEP support to HVM guest

Intel new CPU supports SMEP (Supervisor Mode Execution Protection). SMEP
prevents software operating with CPL < 3 (supervisor mode) from fetching
instructions from any linear address with a valid translation for which the U/S
flag (bit 2) is 1 in every paging-structure entry controlling the translation
for the linear address.

This patch adds SMEP support to HVM guest.

Signed-off-by: Yang Wei <wei.y.yang@intel.com>
Signed-off-by: Shan Haitao <haitao.shan@intel.com>
Signed-off-by: Li Xin <xin.li@intel.com>
Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>