Use DBUS instead of pid file to prevent multiple simultaneous instances

The pid file solution was racy. Since we register DBUS name anyway, we
can use it for this purpose.

qubesmanager/main.py

@@ -2002,23 +2002,15 @@ def sighup_handler(signum, frame):
 
 
 def main():
-    # Avoid starting more than one instance of the app
-    lock = QubesDaemonPidfile("qubes-manager")
-    if lock.pidfile_exists():
-        if lock.read_pid() == os.getpid():
-            pass
-        elif lock.pidfile_is_stale():
-            lock.remove_pidfile()
-            print "Removed stale pidfile " \
-                  "(has the previous daemon instance crashed?)."
-        else:
-            show_running_manager_via_dbus()
-            exit(0)
-
-    lock.create_pidfile()
-
     signal.signal(signal.SIGHUP, sighup_handler)
 
+    global system_bus
+    system_bus = QDBusConnection.systemBus()
+    # Avoid starting more than one instance of the app
+    if not system_bus.registerService('org.qubesos.QubesManager'):
+        show_running_manager_via_dbus()
+        return
+
     global qubes_host
     qubes_host = QubesHost()
 
@@ -2065,9 +2057,6 @@ def main():
     wm.add_watch(os.path.dirname(table_widgets.qubes_dom0_updates_stat_file),
                  EventsCodes.OP_FLAGS.get('IN_CREATE'))
 
-    global system_bus
-    system_bus = QDBusConnection.systemBus()
-    system_bus.registerService('org.qubesos.QubesManager')
     system_bus.registerObject(dbus_object_path, manager_window)
 
     trayIcon.show()
@@ -2075,7 +2064,6 @@ def main():
     show_manager()
     app.exec_()
 
-    lock.remove_pidfile()
     trayIcon = None