Fix database poison warnings, part 2

The following warning appears in the unit test logs a number of times.
This patch fixes some of those warnings by stubbing at the object layer
rather than the database layer.

    "UserWarning: This test uses methods that set internal oslo_db
state, but it does not claim to use the database. This will conflict
with the setup of tests that do use the database and cause failures
later."

Note that this warning is only emitted once per unit test worker, so new
offenders will show up in the logs each time you fix a test until they
are all gone.

Change-Id: I29dca2fd07c7b106a51fb1608caa87b1a5548863
Related-Bug: #1568414

nova/tests/unit/api/openstack/compute/test_volumes.py

@@ -37,6 +37,7 @@ import nova.conf
 from nova import context
 from nova import exception
 from nova import objects
+from nova.objects import base
 from nova import test
 from nova.tests.unit.api.openstack import fakes
 from nova.tests.unit import fake_block_device
@@ -103,8 +104,9 @@ def fake_compute_volume_snapshot_create(self, context, volume_id,
     pass
 
 
-def fake_bdms_get_all_by_instance(context, instance_uuid, use_slave=False):
-    return [fake_block_device.FakeDbBlockDeviceDict(
+@classmethod
+def fake_bdm_list_get_by_instance_uuid(cls, context, instance_uuid):
+    db_list = [fake_block_device.FakeDbBlockDeviceDict(
             {'id': 1,
              'instance_uuid': instance_uuid,
              'device_name': '/dev/fake0',
@@ -124,6 +126,8 @@ def fake_bdms_get_all_by_instance(context, instance_uuid, use_slave=False):
              'snapshot_id': None,
              'volume_id': FAKE_UUID_B,
              'volume_size': 1})]
+    item_cls = objects.BlockDeviceMapping
+    return base.obj_make_list(context, cls(), item_cls, db_list)
 
 
 class BootFromVolumeTest(test.TestCase):
@@ -357,8 +361,9 @@ class VolumeAttachTestsV21(test.NoDBTestCase):
 
     def setUp(self):
         super(VolumeAttachTestsV21, self).setUp()
-        self.stub_out('nova.db.block_device_mapping_get_all_by_instance',
-                      fake_bdms_get_all_by_instance)
+        self.stub_out('nova.objects.BlockDeviceMappingList'
+                      '.get_by_instance_uuid',
+                      fake_bdm_list_get_by_instance_uuid)
         self.stubs.Set(compute_api.API, 'get', fake_get_instance)
         self.stubs.Set(cinder.API, 'get', fake_get_volume)
         self.context = context.get_admin_context()