nixos/libvirtd: always start libvirtd

Previously we did socket-activation but this breaks the autostart
feature since upstream expects libvirtd to be started unconditionally on
boot.

Fixes #171623.

nixos/modules/virtualisation/libvirtd.nix

@@ -336,6 +336,7 @@ in
     };
 
     systemd.services.libvirtd = {
+      wantedBy = [ "multi-user.target" ];
       requires = [ "libvirtd-config.service" ];
       after = [ "libvirtd-config.service" ]
         ++ optional vswitch.enable "ovs-vswitchd.service";