core: prevent rollback on VmReplicateDiskFinish success

Preventing rollback on execution success
(moved rollback avoidance logic to revertTask).

Change-Id: I06a8d395c729e915ece4e0ff413cb006542d4785
Bug-Url: https://bugzilla.redhat.com/923992
Signed-off-by: Daniel Erez <derez@redhat.com>