docs: document how to disable memballoon

https://bugzilla.redhat.com/show_bug.cgi?id=623903 documents a qemu
bug that causes libvirt to hang if virt-manager happens to be
querying balloon info when a guest is paused.  Until the qemu bug
is fixed, people need to know how to avoid the issue.

* docs/formatdomain.html.in (Memory balloon device): Mention
model='none'.

docs/formatdomain.html.in

@@ -1646,6 +1646,9 @@ qemu-kvm -net nic,model=? /dev/null
       need to explicitly add this element in the guest XML unless a
       specific PCI slot needs to be assigned.
       <span class="since">Since 0.8.3, Xen, QEMU and KVM only</span>
+      Additionally, <span class="since">since 0.8.4</span>, if the
+      memballoon device needs to be explicitly disabled,
+      <code>model='none'</code> may be used.
     </p>
 
     <p>