qemu: Prohibit chaning affinity of domain process if placement is 'auto'

On one hand, numad probably will manage the affinity of domain process
dynamically in future. On the other hand, even numad won't manage it,
it still could confusion. Let's make things simpler enough to avoid
the lair for now.

src/qemu/qemu_driver.c

@@ -4204,6 +4204,13 @@ qemudDomainPinEmulator(virDomainPtr dom,
         goto cleanup;
     }
 
+    if (vm->def->placement_mode == VIR_DOMAIN_CPU_PLACEMENT_MODE_AUTO) {
+        virReportError(VIR_ERR_OPERATION_INVALID, "%s",
+                       _("Changing affinity for emulator thread dynamically "
+                         "is not allowed when CPU placement is 'auto'"));
+        goto cleanup;
+    }
+
     if (virDomainLiveConfigHelperMethod(driver->caps, vm, &flags,
                                         &persistentDef) < 0)
         goto cleanup;