core: Disallow moving SD to maintanance if there are async tasks

Added a check whether there are running tasks on the storage domain
to the canDo section of the decativate storage domain command, before this
patch the test would have occur only for domains of type master/import and
not data domain

Change-Id: Iffb08a180be66ce74d70761ce75db96f72fe93e6
Bug-Url: https://bugzilla.redhat.com/953455
Signed-off-by: Tal Nisan <tnisan@redhat.com>