New C only API: guestfs_event_to_string.
ocaml: Add binding for Guestfs.event_to_string and use it in events test.
fish: Use guestfs_event_to_string instead of guestfish-specific generated functions.
java: Use guestfs_event_to_string instead of generated code.
perl: Add binding for guestfs_event_to_string.
python: Add binding for guestfs_event_to_string.
ruby: Add binding for guestfs_event_to_string.