cpu: Better support for ppc64 compatibility modes
cpu: Better support for ppc64 compatibility modes

Not all combinations of host CPU models and compatibility modes
are valid, so we need to make sure we don't try to do something
that QEMU will reject.

Moreover, we need to apply a different logic to guests using
host-model and host-passthrough modes when testing them for host
compatibility.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1251927

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