And it's true, there is no package qemu installed.  So, I presume that
dnf found the package qemu at virt-preview, because the repository is
enabled, but that it is not installed.  What was the list command you
used?  dnf list?  Or dnf list installed?  Or dnf list available?  If
you used just dnf list, then it shows both packages that are installed
and available in repositories for installation.

That makes sense.  And it obviously worked!  And I'm not saying you
should never use third party repositories, just to realize that if you
do, you could have problems.

So what does dnf say when you run
dnf -v upgrade qemu\*
?

>From what you've posted, there is no reason that it shouldn't upgrade
the qemu packages, if there are updates available.  If the qemu package
at the virt-preview site is blocking the update because 2.5.-6 is
considered newer than the update packages version, you could turn off
the virt-preview repository in /etc/yum.repos.d, run dnf clean
metadata, run the transaction, and then turn virt-preview back on.