Strengthen deps

Example autopkgtest failure: oz. You can clearly see how the test assumes that installing libvirt-daemon, even with Recommends disabled, will result in the QEMU driver being available, and that is no longer the case after the recent changes.

Problems also arise on systems where installing Recommends is disabled, and although that's probably not a very common scenario, it is nonetheless a valid configuration in which we should ensure a smooth upgrade.

I have tested upgrade from stable and everything seems to be working nicely. The main drawback of this approach is the fact that we'll have to wait one more release until drivers and configuration files can be selected in a more granular fashion, but that's not any worse than what we had before, so I can live with that; a smaller one is that you might end up installing a driver or some configuration without also installing the daemon, but that's just a minor annoyance and ultimately completely harmless.

Once bullseye is out and we no longer have to worry about upgrading from buster, we can get back to the more flexible arrangement we had before, making sure we file bugs against packages such as oz as well so that they can start explicitly depending on the various drivers.

Marked as WIP because I haven't yet edited the description for libvirt-daemon-config-* and libvirt-login-shell, or created NEWS entries that correct the existing ones when it comes to it being possible to remove these new packages after upgrade: if the changes to debian/control look reasonable, I will do that next.

@agx @paelzer-guest please take a look and let me know what you think!