cpu: CPU model names have to match on ppc64

cpu: CPU model names have to match on ppc64

Limitations of the POWER architecture mean that you can't run
eg. a POWER7 guest on a POWER8 host when using KVM. This applies
to all guests, not just those using VIR_CPU_MATCH_STRICT in the
CPU definition; in fact, exact and strict CPU matching are
basically the same on ppc64.

This means, of course, that hosts using different CPUs have to be
considered incompatible as well.

Change ppc64Compute(), called by cpuGuestData(), to reflect this
fact and update test cases accordingly.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1250977