Fix segfault when starting a domain with no cpu definition

My commit fba6bc47 iterated over the features in cpu definition
without checking if there is one.

src/qemu/qemu_migration.c

@@ -1513,7 +1513,7 @@ qemuMigrationIsAllowed(virQEMUDriverPtr driver, virDomainObjPtr vm,
         return false;
     }
 
-    for (i = 0; i < def->cpu->nfeatures; i++) {
+    for (i = 0; def->cpu && i < def->cpu->nfeatures; i++) {
         virCPUFeatureDefPtr feature = &def->cpu->features[i];
 
         if (feature->policy != VIR_CPU_FEATURE_REQUIRE)

src/qemu/qemu_process.c

@@ -3636,7 +3636,7 @@ qemuProcessVerifyGuestCPU(virQEMUDriverPtr driver, virDomainObjPtr vm)
             }
         }
 
-        for (i = 0; i < def->cpu->nfeatures; i++) {
+        for (i = 0; def->cpu && i < def->cpu->nfeatures; i++) {
             virCPUFeatureDefPtr feature = &def->cpu->features[i];
 
             if (feature->policy != VIR_CPU_FEATURE_REQUIRE)