[6.8.0-RC6] CPU Pinning Editor removes VM template if an XML error occurs

Running RC5 vs RC6 because of the USB device passthrough issue mentioned in another bug report.

I had a VM (Cam 1) with a custom CPU block with 16 CPU's pinned. This is to enable hyper-threading in the VM when the Threadripper CPU itself doesn't really support it.

  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>EPYC</model>
    <topology sockets='1' cores='14' threads='2'/>
    <cache level='3' mode='emulate'/>
    <feature policy='require' name='topoext'/>
    <feature policy='disable' name='monitor'/>
    <feature policy='require' name='hypervisor'/>
    <feature policy='disable' name='svm'/>
    <feature policy='disable' name='x2apic'/>
    <numa>
      <cell id='0' cpus='0-13' memory='6291456' unit='KiB'/>
      <cell id='1' cpus='14-27' memory='6291456' unit='KiB'/>
    </numa>
  </cpu>

I cloned the hard drive of another Windows 10 VM and used "Settings > CPU Pinning" to remove four CPU's (2 CPU/2 HT) from "Cam 1" and assigned them to "Cam 1 PiP". The VM "Cam 1 PiP" had a standard web GUI form editor template. When I clicked the Apply button, I got back an error message akin to "CPU topology doesn't match maximum vcpu count" and then the "Cam 1" VM was gone from the list of configured VM's.

I recreated the "Cam 1" XML but I wasn't able to duplicate.

Recommend restoring the original XML if the CPU Pinning edit of the XML results in an invalid XML.

vm1-diagnostics-20191119-0743.zip