webadmin: add disk - cleanup and duplicate code removal

Add/Edit disk dialog:
* Removing duplicate code from DiskListModel/VmDiskListModel/VmGuideModel.
* Moving add/edit logic to DiskModel for single point of failure
  (Initialize method is used by ListModels that invoke the dialog).
* Cleaning StartProgress/StopProgress calls by subscribing to
  Frontend query start/complete service (using Frontend.Subscribe and
  getHash() for each query.
* Converting asynchronous fetch of configuration values to synchronous.

Change-Id: Id10fa9f34c9c0c42cb91394569196c813f4e2d18
Signed-off-by: Daniel Erez <derez@redhat.com>