qemu: fix CPU model error probing capabilities for ppc

qemu: fix CPU model error probing capabilities for ppc

The CPU driver only supports CPU models for PPC64 architecture, not
plain PPC.

  Failed to probe capabilities for /usr/bin/qemu-system-ppc:
  this function is not supported by the connection driver:
  'ppc' architecture is not supported by CPU driver

This fixes a bug in

  commit db873ab3bc5f6fbd7d227727a62ccfee3d1c3e9d
  Author: Jiri Denemark <jdenemar@redhat.com>
  Date:   Thu May 17 17:08:42 2018 +0200

    qemu: Adapt to changed ppc64 CPU model names

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>