domain_conf: Format <pvpanic/> without address correctly

We have something like pvpanic device. However, in some cases it does
not have any address assigned, in which case we produce this ugly XML
(still valid though):

  <devices>
    <emulator>/usr/bin/qemu</emulator>
    ...
    <panic>
    </panic>
  </devices>

Lets format "<panic/>" instead.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/conf/domain_conf.c

@@ -18840,13 +18840,21 @@ virDomainWatchdogDefFormat(virBufferPtr buf,
 static int virDomainPanicDefFormat(virBufferPtr buf,
                                    virDomainPanicDefPtr def)
 {
-    virBufferAddLit(buf, "<panic>\n");
-    virBufferAdjustIndent(buf, 2);
-    if (virDomainDeviceInfoFormat(buf, &def->info, 0) < 0)
-        return -1;
-    virBufferAdjustIndent(buf, -2);
-    virBufferAddLit(buf, "</panic>\n");
+    virBuffer childrenBuf = VIR_BUFFER_INITIALIZER;
+    int indent = virBufferGetIndent(buf, false);
 
+    virBufferAddLit(buf, "<panic");
+    virBufferAdjustIndent(&childrenBuf, indent + 2);
+    if (virDomainDeviceInfoFormat(&childrenBuf, &def->info, 0) < 0)
+        return -1;
+    if (virBufferUse(&childrenBuf)) {
+        virBufferAddLit(buf, ">\n");
+        virBufferAddBuffer(buf, &childrenBuf);
+        virBufferAddLit(buf, "</panic>\n");
+    } else {
+        virBufferAddLit(buf, "/>\n");
+    }
+    virBufferFreeAndReset(&childrenBuf);
     return 0;
 }
 

tests/qemuxml2argvdata/qemuxml2argv-panic-no-address.args

+LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test QEMU_AUDIO_DRV=none \
+/usr/bin/qemu -S -M pc -m 214 -smp 1 -nographic -nodefconfig -nodefaults \
+-monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -usb \
+-hda /dev/HostVG/QEMUGuest1 \
+-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3 -device pvpanic

tests/qemuxml2argvdata/qemuxml2argv-panic-no-address.xml

+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219136</memory>
+  <currentMemory unit='KiB'>219136</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <disk type='block' device='disk'>
+      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
+    </disk>
+    <controller type='usb' index='0'/>
+    <controller type='fdc' index='0'/>
+    <controller type='ide' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
+    <memballoon model='virtio'/>
+    <panic/>
+  </devices>
+</domain>

tests/qemuxml2argvtest.c

@@ -1522,6 +1522,9 @@ mymain(void)
     DO_TEST("panic", QEMU_CAPS_DEVICE_PANIC,
             QEMU_CAPS_DEVICE, QEMU_CAPS_NODEFCONFIG);
 
+    DO_TEST("panic-no-address", QEMU_CAPS_DEVICE_PANIC,
+            QEMU_CAPS_DEVICE, QEMU_CAPS_NODEFCONFIG);
+
     DO_TEST("fips-enabled", QEMU_CAPS_ENABLE_FIPS);
 
     DO_TEST("shmem", QEMU_CAPS_PCIDEVICE,

tests/qemuxml2xmltest.c

@@ -401,6 +401,7 @@ mymain(void)
     DO_TEST("pcihole64-q35");
 
     DO_TEST("panic");
+    DO_TEST("panic-no-address");
 
     DO_TEST_DIFFERENT("disk-backing-chains");