conf: only format <controller> as a pair tag when needed

Make the decision based on the usage of childBuf buffer.

This fixes the oddity in the test case introduced by commit c1c4d0d5
where we would format an empty pair tag.

src/conf/domain_conf.c

@@ -21477,7 +21477,6 @@ virDomainControllerDefFormat(virBufferPtr buf,
         def->queues || def->cmd_per_lun || def->max_sectors || def->ioeventfd ||
         def->iothread ||
         virDomainDeviceInfoNeedsFormat(&def->info, flags) || pcihole64) {
-        virBufferAddLit(buf, ">\n");
 
         if (pciModel) {
             modelName = virDomainControllerPCIModelNameTypeToString(def->opts.pciopts.modelName);
@@ -21526,7 +21525,10 @@ virDomainControllerDefFormat(virBufferPtr buf,
             virBufferAsprintf(&childBuf, "<pcihole64 unit='KiB'>%lu</"
                               "pcihole64>\n", def->opts.pciopts.pcihole64size);
         }
+    }
 
+    if (virBufferUse(&childBuf)) {
+        virBufferAddLit(buf, ">\n");
         virBufferAddBuffer(buf, &childBuf);
         virBufferAddLit(buf, "</controller>\n");
     } else {

tests/qemuxml2xmloutdata/qemuxml2xmlout-s390-defaultconsole.xml

@@ -14,8 +14,7 @@
   <on_crash>destroy</on_crash>
   <devices>
     <emulator>/usr/bin/qemu-system-s390x</emulator>
-    <controller type='virtio-serial' index='0'>
-    </controller>
+    <controller type='virtio-serial' index='0'/>
     <console type='pty'>
       <target type='virtio' port='0'/>
     </console>