* Do not store source_{volume_id, image_id, snapshot_id} as seperate
  fields. Instead store them in the 'source' field, with a special prefix
  to indicate the source type.
* Add 'origin' field to Volume model to contain the origin that will be
  passed to Ganeti disk
* Check that 'source' option is used only for ext_* volumes
* Check that if a volume is used as source, the volume is in 'AVAILABLE'
  state
* Implement deleting a volume by detaching it from the corresponding
  server. Also, forbid detaching the root volume(index=0) of a server
* Rename 'hotplug' to 'hotplug_if_possible' in Ganeti methods.
* Rename 'name' and 'description' API attributes to 'display_name' and
  'display_description'