Modify LUCreateInstance to support file backend

- Modfiy _GenerateDiskTemplate to support file-based disk template
- Modify _CreateDisks to create directory needed for file-based
  instances before creating the actual files
- Modify _RemoveDisks to delete directory for file-based instances
  after deleting their VBDs
- Add Prereq-check to check if given file-driver is valid
- Add Prereq-check to check if given file-storage-dir path is relative

Reviewed-by: iustinp