First useful solution

.gitignore

@@ -1 +1,4 @@
-bin
+bin/*
+!bin/.keep
+qemu/qcow2/*
+!qemu/qcow2/.keep

appvm.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+if [[ "$1" == "build" && "$2" != "" ]]; then
+    NIX_PATH=$NIX_PATH:. nix-build '<nixpkgs/nixos>' -A config.system.build.vm -I nixos-config=nix/${2}.nix || exit 1
+    NIX_SYSTEM=$(realpath result/system)
+    mkdir -p bin
+    sed "s;NIX_SYSTEM_PLACEHOLDER;${NIX_SYSTEM};" qemu/qemu.template > bin/appvm.${2}
+    sed -i "s;NAME_PLACEHOLDER;${2};" bin/appvm.${2}
+    chmod +x bin/appvm.${2}
+    unlink result
+else
+    echo "Usage: $0 build APPLICATION"
+fi

nix/tdesktop.nix

@@ -2,7 +2,7 @@
 {
   imports = [
     <nixpkgs/nixos/modules/virtualisation/qemu-vm.nix>
-    <base.nix>
+    <nix/base.nix>
   ];
 
   environment.systemPackages = [ pkgs.tdesktop ];

qemu/qemu.template

@@ -1,6 +1,8 @@
 #!/bin/bash
 
-NIX_DISK_IMAGE=$(readlink -f ${NIX_DISK_IMAGE:-./tdesktop.qcow2})
+NAME=NAME_PLACEHOLDER
+
+NIX_DISK_IMAGE=$(readlink -f ${NIX_DISK_IMAGE:-./${NAME}.qcow2})
 
 if ! test -e "$NIX_DISK_IMAGE"; then
     qemu-img create -f qcow2 "$NIX_DISK_IMAGE" 512M || exit 1
@@ -19,7 +21,7 @@ NIX_SYSTEM="NIX_SYSTEM_PLACEHOLDER"
 
 # Start QEMU.
 qemu-system-x86_64 -enable-kvm \
-    -name tdesktop \
+    -name NAME_PLACEHOLDER \
     -m 1024 \
     -smp 1 \
     -device virtio-rng-pci \