Description of problem:
There's a new CPUId bit 80000021/bit 6
'NullSelectClearsBase - Null segment selector loads also clear the destination segment register base and limit'
guests currently detect this behaviour at run time, but there's an upstream fix that starts using this bit and bz 2016959/2016961 request backports; once we start using this bit we should advertise if the cpu has it.
Milan apparently does.
Rome doesn't advertise the bit but is believed to have actually fixed it but not advertise it.

We should advertise the bit in the Milan and Rome models.

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