Virt-manager

Installation

NixOS

virtualisation.libvirtd.enable = true;
programs.dconf.enable = true;
environment.systemPackages = with pkgs; [ virt-manager ];

You will get a warning when you open it for the first time

Could not detect a default hypervisor. Make sure the appropriate virtualization packages containing kvm, qemu, libvirt, etc. are installed and that libvirtd is running.

To resolve

File (in the menu bar) -> Add connection

HyperVisor = QEMU/KVM
Autoconnect = checkmark

Connect

You can get the following error :

authentication unavailable: no polkit agent available to authenticate action 'org.libvirt.unix.manage'

To resolve, add the user to the libvirtd group.

{
  users.users.<myuser>.extraGroups = [ "libvirtd" ];
}