cpu: Don't update host-model guest CPUs on ppc64

If a guest CPU is defined using

  <cpu mode='host-model'/>

the <model> sub-element will contain the compatibility mode to use.
That means we can't just copy the host CPU model on cpuUpdate(),
otherwise we'll overwrite that information and migration of such
guests will fail.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1251927

src/cpu/cpu_ppc64.c

@@ -667,13 +667,13 @@ ppc64DriverUpdate(virCPUDefPtr guest,
                   const virCPUDef *host)
 {
     switch ((virCPUMode) guest->mode) {
-    case VIR_CPU_MODE_HOST_MODEL:
     case VIR_CPU_MODE_HOST_PASSTHROUGH:
         guest->match = VIR_CPU_MATCH_EXACT;
         guest->fallback = VIR_CPU_FALLBACK_FORBID;
         virCPUDefFreeModel(guest);
         return virCPUDefCopyModel(guest, host, true);
 
+    case VIR_CPU_MODE_HOST_MODEL:
     case VIR_CPU_MODE_CUSTOM:
         return 0;