libvirt shouldn't allow an ambiguous XML configuration regarding CPU
topology. Example, what guest CPU config does the following XML represent?

<domain>
  ...
  <vcpus>3</vcpus>
  <cpu>
    <topology sockets='2' cores='2' threads='2'/>
  </cpu>
  ...
</domain>

It is unclear, and libvirt shouldn't accept that config. What I think libvirt
should do is make sockets= a readonly attribute that just mirrors the old style
<vcpus> value. We just throw out the any value the user passes for sockets= and sync with vcpus.

Then we clarify the docs that for HVM, <vcpus> is not 'logical guest
cpus' or anything like that, it represents a distinct socket to the guest. That
should allow people to edit <vcpus> like normal. And my understanding is that
HVs that do CPU hotplug do it at the socket level anyways, so setvcpus commands
still retain the correct semantics.

Some background: https://bugzilla.redhat.com/show_bug.cgi?id=708892#c2