log: Separate thread ID from timestemp in ring buffer

When we write a log message into a log, we separate thread ID from
timestamp using ": ". However, when storing the message into the ring
buffer, we omitted the separator, e.g.:

    2013-02-27 11:49:11.852+00003745: ...

src/util/virlog.c

@@ -850,6 +850,7 @@ virLogVMessage(virLogSource source,
      */
     virLogLock();
     virLogStr(timestamp);
+    virLogStr(": ");
     virLogStr(msg);
     virLogUnlock();
     if (emit == 0)