Refactor: fencing: improve memory handling of fencing event results

Previously, stonith_event_t:opaque was directly set to a local
pcmkaction_result_t variable passed to xml_to_event(). This was rather
brittle, so to lessen the chance of future memory errors, define a struct
containing a pcmkaction_result_t as its only member, and dynamically allocate
it in xml_to_event().