mgmt: send artificial 'connection-established' event just after connection

This allows avoid race condition between  registering event handlers and
performing some action. The important thing is the event sent after
registering event handlers in qubesd. This means state changes (like
VM start/stop) after 'connection-established' event will be included in
event stream.

QubesOS/qubes-issues#2622

qubes/mgmt.py

@@ -583,6 +583,9 @@ class QubesMgmt(AbstractQubesMgmt):
             type(self.app).add_handler('*', handler)
         qubes.vm.BaseVM.add_handler('*', handler)
 
+        # send artificial event as a confirmation that connection is established
+        self.send_event(self.app, 'connection-established')
+
         try:
             yield from wait_for_cancel
         except asyncio.CancelledError:

qubes/tests/mgmt.py

@@ -856,7 +856,10 @@ class TC_00_VMs(MgmtTestCase):
         self.assertEventFired(self.emitter,
             'mgmt-permission:' + 'mgmt.Events')
         self.assertEqual(send_event.mock_calls,
-            [unittest.mock.call(self.vm, 'test-event', arg1='abc')])
+            [
+                unittest.mock.call(self.app, 'connection-established'),
+                unittest.mock.call(self.vm, 'test-event', arg1='abc')
+            ])
 
 
     def test_990_vm_unexpected_payload(self):