virt: allow for direct mounting of LocalBlockImages

Block backed images, are already presented as block devices, so
LocalBlockImages can be mounted directly instead of being linked
with loopback or nbd to another block device.

Change-Id: I5aae1e293832f7cff7bc00d793bcc86ededf5eec
Closes-bug: #1287754

nova/virt/disk/mount/block.py

@@ -0,0 +1,37 @@
+# Copyright 2015 Rackspace Hosting, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+"""Support for mounting block device based images directly."""
+
+from oslo_log import log as logging
+
+from nova.virt.disk.mount import api
+
+LOG = logging.getLogger(__name__)
+
+
+class BlockMount(api.Mount):
+    """Block device backed images do not need to be linked because
+       they are already exposed as block devices and can be mounted
+       directly.
+    """
+    mode = 'block'
+
+    def get_dev(self):
+        self.device = self.image.path
+        self.linked = True
+        return True
+
+    def unget_dev(self):
+        self.linked = False
+        self.device = None