vhost: Rename last_index to vq_index_end

vhost: Rename last_index to vq_index_end

The doc of this field pointed out that last_index is the last vq index.
This is misleading, since it's actually one past the end of the vqs.

Renaming and modifying comment.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20211104085625.2054959-2-eperezma@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>