Fix comment about virCgroupPtr locking rules in QEMU driver

The virCgroupPtr instance APIs are safe to use without locking
in the QEMU driver, since all internal state they rely on is
immutable. Update the comment to reflect this.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/qemu/qemu_conf.h

@@ -164,7 +164,7 @@ struct _virQEMUDriver {
     /* Atomic increment only */
     int nextvmid;
 
-    /* Immutable pointer. Unsafe APIs XXX */
+    /* Immutable pointer. Immutable object */
     virCgroupPtr cgroup;
 
     /* Atomic inc/dec only */