x86/HVM: permit CLFLUSH{,OPT} on execute-only code segments

Both SDM and PM explicitly permit this.

Fixes: 52dba7bd0b ("x86emul: generalize wbinvd() hook")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Paul Durrant <paul@xen.org>

xen/arch/x86/hvm/emulate.c

@@ -2310,7 +2310,9 @@ static int hvmemul_cache_op(
         ASSERT(!is_x86_system_segment(seg));
 
         rc = hvmemul_virtual_to_linear(seg, offset, 0, NULL,
-                                       hvm_access_read, hvmemul_ctxt, &addr);
+                                       op != x86emul_clwb ? hvm_access_none
+                                                          : hvm_access_read,
+                                       hvmemul_ctxt, &addr);
         if ( rc != X86EMUL_OKAY )
             break;