core: fix task revert for ImportRepoImageCommand

ImportRepoImageCreateTaskHandler needs to define getRevertTaskType in
order to have the proper task handling in CommandBase during the task
revert process.

The compensation implementation that was previously using:

  RunVdsCommand(VDSCommandType.DeleteImageGroup, ...)

has been replaced with the more complete:

  runInternalAction(VdcActionType.RemoveDisk, ...)

which handles the entire removal of the disk (database entry included).

Change-Id: I2fea15129eb9e1b0dc1ed6d7d09d8dd63cddd5db
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>