nixos/libvirtd: add qemu-img to $PATH of the daemon

...because daemon's $PATH does not include "/run/current-system/sw/bin"

nixos/modules/virtualisation/libvirtd.nix

···
           dmidecode
           dnsmasq
           ebtables
+          cfg.qemuPackage # libvirtd requires qemu-img to manage disk images
         ]
         ++ optional vswitch.enable vswitch.package;