Fix DeviceUnavailable exception

We were accessing the wrong module (cinderlib) to find the
DeviceUnavailable exception, now we access it in the right place
(cinder.exception).

HISTORY.rst

@@ -27,6 +27,7 @@ History
   - Fix name & description inconsistency
   - Set created_at field on creation
   - Connection fields not being set
+  - DeviceUnavailable exception
 
 0.1.0 (2017-11-03)
 ------------------

cinderlib/cinderlib.py

@@ -346,7 +346,8 @@ class RBDConnector(connectors.rbd.RBDConnector):
         """Verify an existing RBD handle is connected and valid."""
         try:
             self._execute('dd', 'if=' + path, 'of=/dev/null', 'bs=4096',
-                          'count=1', run_as_root=True)
+                          'count=1', root_helper=self._root_helper,
+                          run_as_root=True)
         except putils.ProcessExecutionError:
             return False
         return True

cinderlib/objects.py

@@ -22,6 +22,7 @@ from cinder import context
 # having all the other imports as they could change.
 from cinder.cmd import volume as volume_cmd
 from cinder import objects as cinder_objs
+from cinder import exception as cinder_exception
 from cinder.objects import base as cinder_base_ovo
 from os_brick import exception as brick_exception
 from os_brick import initiator as brick_initiator
@@ -681,7 +682,7 @@ class Connection(Object):
             LOG.exception('Could not validate device %s', self.path)
 
         if unavailable:
-            raise exception.DeviceUnavailable(
+            raise cinder_exception.DeviceUnavailable(
                 path=self.path, attach_info=self._ovo.connection_information,
                 reason=('Unable to access the backend storage via path '
                         '%s.') % self.path)