vl: Improve legibility of BlockdevOptions queue

Give the queue head type a name: BlockdevOptionsQueue.

Rename the queue entry type from BlockdevOptions_queue to
BlockdevOptionsQueueEntry.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190308131445.17502-6-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>