ppc 'max' CPU model is unlike the other targets 'max' CPU model
Description of problem:
On most targets the 'max' CPU model is either equivalent to 'host' (for KVM) or equivalent to all available CPU features (for TCG).

On PPC64, however, this is not the case. Instead the 'max' model is an alias of the old '7400_v2.9' and simply doesn't work.

This is confusing. At the very least the 'max' model alias should be deleted. Ideally a proper replacement would be introduced that matches semantics on other arches.
Steps to reproduce:
1. qemu-system-ppc64 -cpu max

should be equiv to '-cpu host' or should expose all TCG features.