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 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>

src/qemu/qemu_capabilities.c

@@ -2454,7 +2454,7 @@ virQEMUCapsFetchCPUDefinitions(qemuMonitorPtr mon,
 
     /* QEMU 2.11 for Power renamed all CPU models to lower case, we need to
      * translate them back to libvirt's upper case model names. */
-    if (ARCH_IS_PPC(arch)) {
+    if (ARCH_IS_PPC64(arch)) {
         VIR_AUTOSTRINGLIST libvirtModels = NULL;
         char **name;