Hello.
Here: https://alt.fedoraproject.org/prerelease/index.html
The links to Cloud Base Images points to Alpha-1.3 leading to a 404 Not Found page. While the right ones should be Alpha-1.7, for instanche: Fedora-Cloud-Base-Vagrant-26_Alpha-1.7.x86_64.vagrant-libvirt.box 
Isn't it?

Thanks.
A.