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 db873ab3
  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: NDaniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>