control: Strengthen dependencies between packages
When splitting a package, such as we did when we moved the QEMU driver from libvirt-daemon to libvirt-daemon-driver-qemu, you need to maintain at least some level of dependency between the original package and the new one, or apt will simply not know that it needs to install the latter in an upgrade scenario. Using Recommends works for most cases, since Debian systems are configured to install those by default; however, there are a couple of cases where it's better to have a stronger dependency. More specifically, due to the way libvirt has been packaged until now, people expect that installing the libvirt-daemon package will result in the QEMU driver being available, and that installing the libvirt-daemon-system package will result in the default network and default set of nwfilters being configured. The stronger dependencies help both for upgrades and in the context of other packages' autopkgtests; unfortunately, it also makes the package less flexible. We're going turn these Depends back into Recommends once bullseye is out.