qemu: command: Don't allow setting 'group_name' alone

The disk tuning group parameter is ignored by qemu if no other
throttling options are set. Reject such configuration, since the name
would not be honored after setting parameters via the live tuning API.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1433180

src/qemu/qemu_command.c

@@ -1259,12 +1259,23 @@ qemuCheckDiskConfigBlkdeviotune(virDomainDiskDefPtr disk,
     }
 
     /* block I/O group 2.4 */
-    if (disk->blkdeviotune.group_name &&
-        !virQEMUCapsGet(qemuCaps, QEMU_CAPS_DRIVE_IOTUNE_GROUP)) {
-        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
-                       _("the block I/O throttling group parameter is "
-                         "not supported with this QEMU binary"));
-        return -1;
+    if (disk->blkdeviotune.group_name) {
+        if (!virQEMUCapsGet(qemuCaps, QEMU_CAPS_DRIVE_IOTUNE_GROUP)) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("the block I/O throttling group parameter is "
+                             "not supported with this QEMU binary"));
+            return -1;
+        }
+
+        /* group_name by itself is ignored by qemu */
+        if (!qemuDiskConfigBlkdeviotuneHasBasic(disk) &&
+            !qemuDiskConfigBlkdeviotuneHasMax(disk) &&
+            !qemuDiskConfigBlkdeviotuneHasMaxLength(disk)) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("group_name can be configured only together with "
+                             "settings"));
+            return -1;
+        }
     }
 
     /* block I/O throttling length 2.6 */