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> The instance can also be migrated between 2 Westmere hosts.
> As I outline in another email, I suspect (perhaps incorrectly) as the VM has "cpu mode='host-model'" in its running config, 
> libvirt may be comparing source and destination host CPUs and not guest and destination CPUs.
 
Indeed it very much seems like it will look at the host cpu-model and not the guest since it is complaining about Broadwell CPU features.
The libvirt documentation also sort of suggests this is the case:
https://libvirt.org/formatdomain.html#elementsCPU
“Prior to libvirt 3.2.0 and QEMU 2.9.0 detection of the host CPU model via QEMU is not supported. Thus the CPU configuration created using host-model may not work as expected.”

You can have a look cheating your way around it.
There is some code which removes the checks which you might be able to (ab)use:
https://bugs.launchpad.net/nova/+bug/1588003

You might want to double-check if a (hard)-rebooted migrated (with the host-model config) instances on the Broadwell compute node does not get a Broadwell cpu before modifying this ;)

Cheers,
Robert van Leeuwen