Comment 6 for bug 1820291

Without such fixed it looks like this:
dpkg: warning: qemu-guest-agent: conffile '/etc/qemu/fsfreeze-hook' is not a plain file or symlink (= '/etc/qemu/fsfreeze-hook')

And then leaves a dpkg-new file:
ubuntu@disco:~$ ll /etc/qemu
total 20
drwxr-xr-x 4 root root 4096 Mar 18 09:34 ./
drwxr-xr-x 87 root root 4096 Mar 18 09:34 ../
drwxr-xr-x 2 root root 4096 Mar 18 09:34 fsfreeze-hook/
drwxr-xr-x 2 root root 4096 Feb 19 05:43 fsfreeze-hook.d/
-rwxr-xr-x 1 root root 1274 Dec 11 17:44 fsfreeze-hook.dpkg-new*

Using mv_conffile now but I'm afraid even that might not be enough, I started a discussion on IRC to poll for best practices for that specific case