x86/hvm: allow guest to use clflushopt and clwb

x86/hvm: allow guest to use clflushopt and clwb

Pass CPU features CLFLUSHOPT and CLWB into HVM domain so that those two
instructions can be used by guest.

The specification of above two instructions can be found in
https://software.intel.com/sites/default/files/managed/0d/53/319433-022.pdf

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Wei Liu <wei.liu2@citrix.com> for tools bits