hsm: use _hsmSchedule for spmStart

Method spmStart can now take advantage of _hsmSchedule to schedule its
job (without persistency).
The spmStart task never needed any persistency because its lifecycle
is strictly tied to the vdsm process and it doesn't involve any storage
modification that requires a rollback (or rollforward).

Change-Id: I578f3bc2360e291b05b38e64e8c8bad87e5cd753
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26772
Reviewed-by: Dan Kenigsberg <danken@redhat.com>