service view: fix stale stop/restart button enabled behavior

fix an issue where the stop or restart button stays enabled for
elements in 'startOnlyServices' after switching selection from a
service that was not restricted to start-only

note: front-end change only as the backend already refused to stop a
start-only service.

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>