rpc: daemon: Fix virtlog/virtlock daemon reload

Trying to reload/SIGUSR1 virtlogd or virtlockd fails with:

error : virNetDaemonRun:747 : internal error: Not all servers restored, cannot run server

Commit 252610f7 changed the daemon state json to allow tracking
multiple servers. However it missed clearing dmn->srvObject after
the json is empty, like the previous code paths handled.  Later on in
virNewDaemonRun, dmn->srvObject is expected to be empty otherwise we
throw the above error.

https://bugzilla.redhat.com/show_bug.cgi?id=1311013
(cherry picked from commit 9b69f022)

src/rpc/virnetdaemon.c

@@ -293,6 +293,10 @@ virNetDaemonAddServerPostExec(virNetDaemonPtr dmn,
             goto error;
         }
 
+        if (virJSONValueObjectKeysNumber(dmn->srvObject) == 0) {
+            virJSONValueFree(dmn->srvObject);
+            dmn->srvObject = NULL;
+        }
     }
 
     srv = virNetServerNewPostExecRestart(object,