discover/event: Ensure event struct exists for async callers

When handling an event, user_event_handle_message() creates an event
struct with relevant parameters. Once user_event_handle_message() is
finished it frees the struct.
However in the case of a dhcp or add_url event, asynchronous jobs may be
spawned that will later reference the event struct. In particular this
becomes a problem when pxe_process_pair() handles an IPAPPEND name/value
pair and tries to access event->device.

In the case of dhcp and add_url events, we avoid this by changing the
event struct's talloc parent to the discover_context struct which
persists until all async pxe jobs have completed.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>