x86/hvm: add support for pcommit instruction

x86/hvm: add support for pcommit instruction

Pass PCOMMIT CPU feature into HVM domain. Currently, we do not intercept
pcommit instruction for L1 guest, and allow L1 to intercept pcommit
instruction for L2 guest.

The specification of pcommit instruction 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>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Wei Liu <wei.liu2@citrix.com> for tools bits