x86emul: simplify prefix handling for VMFUNC

LOCK prefixes get dealt with elsewhere and 66, F2, and F3 can all be
checked for in one go by looking at vex.pfx.

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
@@ -3942,8 +3942,8 @@ x86_emulate(
             goto rdtsc;
         }
         case 0xd4: /* vmfunc */
-            generate_exception_if(lock_prefix | rep_prefix() | (vex.pfx == vex_66),
-                                  EXC_UD, -1);
+            if ( vex.pfx )
+                break;
             fail_if(ops->vmfunc == NULL);
             if ( (rc = ops->vmfunc(ctxt) != X86EMUL_OKAY) )
                 goto done;