This epic tracks using the improved hypervior aware CPU API in libvirt
to make more accurate selections accounting forwhat features can and cannot be virutaldised. this will improve the compatibility fo VM migrations between hosts of diffenrt CPU models.

Problem
-------

The current CPU configuration libvirt APIs that Nova uses, compareCPU()
and baselineCPU(), are "not very useful" (quoting the cover letter[1] of
the libvirt patch series that introduced better APIs), because they
don't consider the capabilities of the "host hypervisor" (KVM, QEMU and
details libvirt knows about the host).

More concretely, with compareCPU() and baselineCPU(), there is no way to
ask if a given CPU model plus CPU flags combination is supported by KVM
and a specific QEMU binary on the host. E.g. "is the combination of
IvyBridge CPU model plus CPU flags 'pcid' and 'ssbd' supported by the
host hypervisor?"

And Nova needs a way to do fine-grained validation of a certain CPU
model plus CPU flags against a specific QEMU binary (and KVM) to allow
well-informed guest CPU configuration decisions.

Proposal
--------

To solve the above problem, upstream libvirt (since 4.0.0, which
requires QEMU 2.9 or above for x86_64) has introduced[2] two new CPU
configuration APIs[1]:

baselineHypervisorCPU()
compareHypervisorCPU()

As their names indicate, these APIs are designed to take into account
the capabilities of the host hypervisor.

Rework Nova's CPU selection logic to incorporate these
"hypervisor-literate" CPU configuration APIs. This will allow for more
effective and reliable guest CPU configuration.

[1] https://www.redhat.com/archives/libvir-list/2018-May/msg01204.html
– New CPU related APIs

[2] https://bugzilla.redhat.com/show_bug.cgi?id=1559832 – [RFC]
– Fine-grained API to validate if a given CPU model and flags are
supported by QEMU / KVM