This is how I got around problem that occurred when I was updating a virtualised Ubuntu 16.04 instance that I have. My usual way to do this is using apt-get or apt from the command line and the process halted because a pre-removal script for the upgrade of avahi-dnsconf failed. The cause was its not disabling the avahi daemon beforehand so I need to execute the following command before repeating the operation:
sudo systemctl disable avahi-daemon
Once the upgrade had completed, then it was time to re-enable the service using the following command:
sudo systemctl enable avahi-daemon
Ideally, this would completed without such manual intervention and there is a bug report for the unexpected behaviour. Hopefully, it will be sorted soon but these steps will fix things for now.