STRUCT_FMT is generic enough, rename it to TYPE_FMT, use it for unions.

Rename COMMAND_FMT to MSG_FMT, since it applies to both commands and
events.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170125130308.16104-2-marcandre.lureau@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>