tests: even more agressive cleanup in tearDown

Remove some more references to objects holding (possibly indirectly)
reference to libvirt connection:
 - local variables in tearDown function
 - running Admin API calls (especially admin.Events)
 - vmm._libvirt_conn directly, in case some reference to Qubes()
   is still there
 - any instance attribute that is an object from 'qubes' python package
   (instead of just those descending from BaseVM)
 - do not create new Qubes() instance for removing VMs - if we already
   have one in self.app

Then trigger garbage collector to really cleanup those objects (and
close relevant file descriptors). It's important do do this before
closing event loop, because some of descructors may try to use it (for
example remove registered handlers).

qubes/tests/__init__.py

@@ -48,6 +48,7 @@ import unittest
 import warnings
 from distutils import spawn
 
+import gc
 import lxml.etree
 import pkg_resources
 
@@ -661,25 +662,42 @@ class SystemTestCase(QubesTestCase):
         self.remove_test_vms()
 
         # close the servers before super(), because that might close the loop
+        server = None
         for server in self.qubesd:
             for sock in server.sockets:
                 os.unlink(sock.getsockname())
             server.close()
+        del server
+
+        # close all existing connections, especially this will interrupt
+        # running admin.Events calls, which do keep reference to Qubes() and
+        # libvirt connection
+        conn = None
+        for conn in qubes.api.QubesDaemonProtocol.connections:
+            if conn.transport:
+                conn.transport.abort()
+        del conn
 
         self.loop.run_until_complete(asyncio.wait([
             server.wait_closed() for server in self.qubesd]))
+        del self.qubesd
 
-        super(SystemTestCase, self).tearDown()
-        # remove all references to VM objects, to release resources - most
-        # importantly file descriptors; this object will live
+        # remove all references to any complex qubes objects, to release
+        # resources - most importantly file descriptors; this object will live
         # during the whole test run, but all the file descriptors would be
         # depleted earlier
+        self.app.vmm._libvirt_conn = None
         del self.app
         del self.host_app
         for attr in dir(self):
-            if isinstance(getattr(self, attr), qubes.vm.BaseVM):
+            obj_type = type(getattr(self, attr))
+            if obj_type.__module__.startswith('qubes'):
                 delattr(self, attr)
 
+        # then trigger garbage collector to really destroy those objects
+        gc.collect()
+
+        super(SystemTestCase, self).tearDown()
 
     def _remove_vm_qubes(self, vm):
         vmname = vm.name
@@ -776,12 +794,14 @@ class SystemTestCase(QubesTestCase):
         # first, remove them Qubes-way
         if os.path.exists(xmlpath):
             try:
-                self.remove_vms(vm for vm in qubes.Qubes(xmlpath).domains
+                try:
+                    app = self.app
+                except AttributeError:
+                    app = qubes.Qubes(xmlpath)
+                self.remove_vms(vm for vm in app.domains
                     if vm.name.startswith(prefix))
-            except (qubes.exc.QubesException, lxml.etree.XMLSyntaxError):
-                # If qubes-test.xml is broken that much it doesn't even load,
-                #  simply remove it. VMs will be cleaned up the hard way.
-                # TODO logging?
+                del app
+            except qubes.exc.QubesException:
                 pass
             os.unlink(xmlpath)