storage: use "systemctl" to stop units, not D-Bus
storage: use "systemctl" to stop units, not D-Bus

With D-Bus, we would have to wait for the job to complete before
continuing with unmountin, etc.  This is too much hair, just call
systemctl, which waits by default for jobs to be complete.

martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit