volume/driver: implement basic snapshot

added basic support for snapshot to VolumeDriver base class.
The implementation is not effective, but works. The effective
implementation should be done by drived driver class.

nova/exception.py

@@ -271,6 +271,10 @@ class SnapshotNotFound(NotFound):
     message = _("Snapshot %(snapshot_id)s could not be found.")
 
 
+class VolumeIsBusy(Error):
+    message = _("deleting volume %(volume_name)s that has snapshot")
+
+
 class ExportDeviceNotFoundForVolume(NotFound):
     message = _("No export device found for volume %(volume_id)s.")