qemu: migration: error if tunnelled + storage specified

Since as the code indicates it doesn't work yet, so let's be
explicit about it.

src/qemu/qemu_migration.c

@@ -1900,13 +1900,14 @@ char *qemuMigrationBegin(virQEMUDriverPtr driver,
     if (flags & (VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC) &&
         virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_NBD_SERVER)) {
         /* TODO support NBD for TUNNELLED migration */
-        if (flags & VIR_MIGRATE_TUNNELLED)
-            VIR_DEBUG("NBD in tunnelled migration is currently not supported");
-        else {
+        if (flags & VIR_MIGRATE_TUNNELLED) {
+            virReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
+                _("NBD in tunnelled migration is currently not supported"));
+            goto cleanup;
+        }
         cookieFlags |= QEMU_MIGRATION_COOKIE_NBD;
         priv->nbdPort = 0;
     }
-    }
 
     if (!(mig = qemuMigrationEatCookie(driver, vm, NULL, 0, 0)))
         goto cleanup;
@@ -2200,17 +2201,12 @@ done:
     if (mig->nbd &&
         flags & (VIR_MIGRATE_NON_SHARED_DISK | VIR_MIGRATE_NON_SHARED_INC) &&
         virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_NBD_SERVER)) {
-        /* TODO support NBD for TUNNELLED migration */
-        if (flags & VIR_MIGRATE_TUNNELLED)
-            VIR_DEBUG("NBD in tunnelled migration is currently not supported");
-        else {
         if (qemuMigrationStartNBDServer(driver, vm, listenAddr) < 0) {
             /* error already reported */
             goto endjob;
         }
         cookieFlags |= QEMU_MIGRATION_COOKIE_NBD;
     }
-    }
 
     if (qemuMigrationBakeCookie(mig, driver, vm, cookieout,
                                 cookieoutlen, cookieFlags) < 0) {