x86emul: shifts/rotates don't allow LOCK prefix

... just like e.g. SHLD/SHRD don't (see commit dee231b5a8 [x86emul:
improve LOCK handling]).

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
@@ -3853,6 +3853,7 @@ x86_emulate(
         break;
 
     case 0xc0 ... 0xc1: grp2: /* Grp2 */
+        generate_exception_if(lock_prefix, EXC_UD);
         switch ( modrm_reg & 7 )
         {
         case 0: /* rol */