GuestDiskStatsInfo's member documentation is parsed as ordinary text
due to missing colons.  The generated documentation shows these
members as "Not documented".

The fix is obvious: add the missing colons.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230425064223.820979-12-armbru@redhat.com>