core: Add the ability to create a dummy task in case of VDSM failure

The following patch provides a helper method to create a dummy task
in case there is VDSM failure when running asynchronous command.
The dummy SPM task will be at ended stage, and the root command that
causes all the tasks will fail (endWithFailure will be executed).

Change-Id: I6e977596af52c81a1aed1b8748a30b9425c62fa3
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>