xen/control: short circuit xctrl_on_watch_event on spurious event

If there's no data to read from xenstore short-circuit
xctrl_on_watch_event to return early, there's no reason to continue
since the lack of data would prevent matching against any known event
type.

Sponsored by: Citrix Systems R&D
MFC with: r352925
MFC after: 1 week