core: Move VDS to Maintenance only if StopSPM is successful

Fixes MaintenanceNumberOfVdssCommand so only VDSs for which StopSPM was
executed successfully, will continues with Move to Maintenance process.

Change-Id: I5c58f9a9629d2e7a496f02c4dececeb842d44543
Bug-Url: https://bugzilla.redhat.com/1023145
Signed-off-by: Martin Perina <mperina@redhat.com>