cpu: Don't update host-model guest CPUs on ppc64
cpu: Don't update host-model guest CPUs on ppc64

If a guest CPU is defined using

  <cpu mode='host-model'/>

the <model> sub-element will contain the compatibility mode to use.
That means we can't just copy the host CPU model on cpuUpdate(),
otherwise we'll overwrite that information and migration of such
guests will fail.

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

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