cpu: Forbid model fallback in the ppc64 driver

cpu: Forbid model fallback in the ppc64 driver

Unlike what happens on x86, on ppc64 you can't mix and match CPU
features to obtain the guest CPU you want regardless of the host
CPU, so the concept of model fallback doesn't apply.

Make sure CPU definitions emitted by the driver, eg. as output of
the cpuBaseline() and cpuUpdate() calls, reflect this fact.