libdrakvuf: event_uid not set for every event handler

[skvl]

drakvuf_trap_info_t.event_uid is set only in two cases:

• pre_mem_cb
• int3_cb

In all other event handlers (e.g. cr3_cb) it remains unset (just 0).

Is this by design? If so we should leave a comment near declaration

[tklengyel]

I would bet it's just an oversight and those were the only events the author of the feature cared about.