Handle InstanceNotFound in libvirt list_instances.

Updates the libvirt driver so that InstanceNotFound is
handled within the list_instances function. This
can occur if an instance is deleted while the
list_instances function is processing.

Fixes LP Bug #1032741.
Change-Id: I97c9533f8b37957e71237c610ac360fbf9f57b9a

nova/tests/test_libvirt.py

@@ -912,6 +912,22 @@ class LibvirtConnTestCase(test.TestCase):
         # Only one should be listed, since domain with ID 0 must be skiped
         self.assertEquals(len(instances), 1)
 
+    def test_list_instances_when_not_found(self):
+
+        def fake_lookup(instance_name):
+            raise exception.InstanceNotFound()
+
+        self.mox.StubOutWithMock(libvirt_driver.LibvirtDriver, '_conn')
+        libvirt_driver.LibvirtDriver._conn.lookupByID = fake_lookup
+        libvirt_driver.LibvirtDriver._conn.numOfDomains = lambda: 1
+        libvirt_driver.LibvirtDriver._conn.listDomainsID = lambda: [0, 1]
+
+        self.mox.ReplayAll()
+        conn = libvirt_driver.LibvirtDriver(False)
+        instances = conn.list_instances()
+        # None should be listed, since we fake deleted the last one
+        self.assertEquals(len(instances), 0)
+
     def test_get_all_block_devices(self):
         xml = [
             # NOTE(vish): id 0 is skipped