Make nova.tests.unit.virt pass with CONF.use_neutron=True by default

The main issue here was if you don't stub out the os-vif plugging
in the libvirt driver, the test_virt_drivers tests will try to
actually execute commands on the system.

Part of blueprint use-neutron-by-default

Change-Id: Iabfc1809d302b80cf4674155feaa0abb61ca8dcb

nova/tests/unit/virt/libvirt/fakelibvirt.py

@@ -1493,6 +1493,8 @@ virConnect = Connection
 class FakeLibvirtFixture(fixtures.Fixture):
     """Performs global setup/stubbing for all libvirt tests.
     """
+    def __init__(self, stub_os_vif=True):
+        self.stub_os_vif = stub_os_vif
 
     def setUp(self):
         super(FakeLibvirtFixture, self).setUp()
@@ -1504,3 +1506,11 @@ class FakeLibvirtFixture(fixtures.Fixture):
             self.useFixture(fixtures.MonkeyPatch(i, sys.modules[__name__]))
 
         disable_event_thread(self)
+
+        if self.stub_os_vif:
+            # Make sure to never try and actually plug VIFs in os-vif unless
+            # we're explicitly testing that code and the test itself will
+            # handle the appropriate mocking.
+            self.useFixture(fixtures.MonkeyPatch(
+                'nova.virt.libvirt.vif.LibvirtGenericVIFDriver._plug_os_vif',
+                lambda *a, **kw: None))

nova/tests/unit/virt/libvirt/test_vif.py

@@ -435,7 +435,7 @@ class LibvirtVifTestCase(test.NoDBTestCase):
 
     def setUp(self):
         super(LibvirtVifTestCase, self).setUp()
-        self.useFixture(fakelibvirt.FakeLibvirtFixture())
+        self.useFixture(fakelibvirt.FakeLibvirtFixture(stub_os_vif=False))
         self.flags(allow_same_net_traffic=True)
         # os_vif.initialize is typically done in nova-compute startup
         os_vif.initialize()