qemu: command: Drop obsolete comment

The change that made qemu not add the memballoon by default happened
prior to 0.12.0. Additionaly the comment was misleading due to the code
that was added below. Since we always need to add a balloon on the
commandline drop the comment.

src/qemu/qemu_command.c

@@ -3526,12 +3526,7 @@ qemuBuildMemballoonCommandLine(virCommandPtr cmd,
                                virQEMUCapsPtr qemuCaps)
 {
     char *optstr;
-    /* QEMU changed its default behavior to not include the virtio balloon
-     * device.  Explicitly request it to ensure it will be present.
-     *
-     * NB: Earlier we declared that VirtIO balloon will always be in
-     * slot 0x3 on bus 0x0
-     */
+
     if (STREQLEN(def->os.machine, "s390-virtio", 10) &&
         virQEMUCapsGet(qemuCaps, QEMU_CAPS_VIRTIO_S390) && def->memballoon)
         def->memballoon->model = VIR_DOMAIN_MEMBALLOON_MODEL_NONE;