qemuMonitorJSONMakeCPUModel: Introduce @hv_passthrough argument

qemuMonitorJSONMakeCPUModel: Introduce @hv_passthrough argument

Apart from setting @migratable prop to the
query-cpu-model-expansion command, we will need @hv-passthrough
so that we can query for expansion of Hyper-V Enlightenments
supported on the current host. The idea is to run:

{
  "execute": "query-cpu-model-expansion",
  "arguments": {
    "type": "full",
    "model": {
      "name": "host",
      "props": {
        "hv-passthrough": true
      }
    }
  }
}

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>