core: add logging to SpmStopVDSCommand

currently SpmStopVdsCommand performs nothing in case of tasks return by
the current spm, this patch just adds logging to that case.

Change-Id: I271b7503270e57f9b08a3272f14905fd38b8d481
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1130766
Signed-off-by: Liron Aravot <laravot@redhat.com>