Windows buildkite agents need AMD/Intel dual configuration

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When running on amdci6, our windows KVM configuration uses the following CPU block:

  <cpu mode='custom' match='exact' check='full'>
    <model fallback='forbid'>EPYC-IBPB</model>
    <feature policy='disable' name='monitor'/>
    <feature policy='require' name='x2apic'/>
    <feature policy='require' name='hypervisor'/>
    <feature policy='require' name='topoext'/>
    <feature policy='require' name='npt'/>
    <feature policy='require' name='nrip-save'/>
  </cpu>

This allows Hyper-V to run, and enables docker to run on the machines.

On the Intel-based AWS c5d.metal instances that we used during the amdci outage period in December 2022, we found that the following CPU block was necessary:

  <cpu mode="custom" match="exact" check="partial">
    <model fallback="allow">IvyBridge</model>
    <feature policy="require" name="hypervisor"/>
    <feature policy="require" name="vmx"/>
  </cpu>

We should parameterize this somehow, perhaps by having two XML templates, or by splicing in pieces together through a xml_template.d directory structure.