Comment 5 for bug 1619064

Working on some of the recent changes I know that a freshly installed clamav does not start the daemon right away. It can't since freshclam hasn't provided any definitions yet.
The clamav-daemon has a condition in its unit file that tests on the availability of such files.

Unfortunately there is no mechanism yet to let the service reevaluate that once freshclam is done, that was part of a discussion in the other bug I remember.

The implications for this bug might be that a service not started due to this might - on a later upgrade run into this I have seen in your log:
  Failed to stop clamav-daemon.socket: Unit clamav-daemon.socket not loaded.

IMHO that stopping in the prerm should be non fatal in case it already (or still) is stopped.

Could you as a try manually start your clamav-daemon (now after a while of freshclam running it should be able to). And then update again. I would hope that then the prerm can shut it down as expected and your update can proceed. No fix yet but a workaround worth trying.