Make vm_ballo optional for libvirt

roles/virtualization/templates/vm.xml.j2

@@ -1,7 +1,7 @@
 <domain type='kvm'>
   <name>{{ hostname }}</name>
   <memory>{{ vm_memory | int * 1024 }}</memory>
-  <currentMemory>{{ vm_ballo | int * 1024 }}</currentMemory>
+  {% if vm_ballo is defined %}<currentMemory>{{ vm_ballo | int * 1024 }}</currentMemory>{% endif %}
   <vcpu placement='static'>{{ vm_cpus }}</vcpu>
   <os>
     <type arch='x86_64' machine="pc-q35-8.0">hvm</type>
@@ -52,4 +52,4 @@
       <model type="virtio" heads="1" primary="yes"/>
     </video>
   </devices>
-</domain>
+</domain>