So far I'm not able to reproduce this on a clean 4.0 install.
I know this might be a dumb question, but what happens when you run these:

sudo apt install -f

and

sudo apt reinstall libvirt-daemon

?

Also, I think I've seen some funny business like this when the local db doesn't think that package exists and/or it is missing from your mirror. Changing the mirror and running 'sudo apt clean && sudo apt update' might also be something good to try. (This is the mirror I'm using if you need it for reference: "http://mishka.snork.ca/")

Just to clarify, all default means here is that if you do not touch "tasksel" during install, you will get XFCE and only XFCE. If you install any other DE's the "update-alternatives" system will set the last one installed by DPKG to default. All this really meanings, in practice, is that the next time "lightdm" (or etc) is started, the drop down will have that latest DE selected. (probably sets it to be the one that loads when issuing "startx" too) It's up to you to select a different DE before hitting enter.

If you don't like the behavior of "update-alternatives" there may be a way to change it. (This is probably why some may have suggested reading man pages)  If you think the default behavior should change, there may be a way to open a bug upstream (Debian)

That said, I get that this is a potential user experience issue, but I don't believe the Devuan developers should be on the hook for changing the way "update-alternatives" works. It is, however, acceptable for them to (pre) check only certain boxes in "tasksel" during install. This is what they've done to select XFCE. (This is what the Debian Project does for the default DE, as well) The Devuan developers focus should be making the INIT systems work well with minimal changes/impact to the behavior to rest of the system.

Remember, we are here because of needless and overreaching changes to the OS, lets be careful not to fall prey to that pitfall as well.