In a discussion spawned from the issue to connect to libvirt through ssh we discussed about using libvirt with the usual default of qemu://session right after installing libvirt* packages as there are some hurdles to pass - those hurdles do not exist as soon as you re-login btw.
What will happen is like:
$ apt install libvirt-daemon-system $ virsh list error: failed to connect to the hypervisor error: Failed to connect socket to '/var/run/libvirt/libvirt-sock': Permission denied
But actually slightly more sinister is when it works but you later miss your guests. Which is due to the default right after install being from the binaries which is qemu:///session (in newer versions where this was changed).
$ apt install libvirt-daemon-system $ virsh uri qemu:///session