util: Fix virCgroupNewMachine ATTRIBUTE_NONNULL args

Commit id 'c3bd0019' removed arg3, but forgot to adjust the numbers
for NONNULL - caused build failure for coverity

src/util/vircgroup.h

@@ -110,7 +110,7 @@ int virCgroupNewMachine(const char *name,
                         int controllers,
                         virCgroupPtr *group)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2)
-    ATTRIBUTE_NONNULL(4);
+    ATTRIBUTE_NONNULL(3);
 
 int virCgroupTerminateMachine(const char *name)
     ATTRIBUTE_NONNULL(1);