No semantic change, but will make it easier for an upcoming patch
to refactor code without having to add forward declarations.  Fix
a poor comment while at it.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180110230825.18321-2-eblake@redhat.com>