x86emul: MOVNTI does not allow 66/F3/F2 prefixes

Intel SDM revision 062 finally makes this explicit.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -6719,6 +6719,7 @@ x86_emulate(
         goto simd_0f_imm8_avx;
 
     case X86EMUL_OPC(0x0f, 0xc3): /* movnti */
+        generate_exception_if(vex.pfx, EXC_UD);
         /* Ignore the non-temporal hint for now. */
         vcpu_must_have(sse2);
         dst.val = src.val;