* New nixos-rebuild action: "nixos-rebuild build-vm" builds a virtual

machine containing a replica (minus the state) of the system
  configuration.  This is mostly useful for testing configuration
  changes prior to doing an actual "nixos-rebuild switch" (or even
  "nixos-rebuild test").  The VM can be started as follows:

  $ nixos-rebuild build-vm
  $ ./result/bin/run-*-vm

  which starts a KVM/QEMU instance.  Additional QEMU options can be
  passed through the QEMU_OPTS environment variable
  (e.g. QEMU_OPTS="-redir tcp:8080::80" to forward a host port to the
  guest).  The fileSystem attribute of the regular system
  configuration is ignored (using mkOverride), because obviously we
  can't allow the VM to access the host's block devices.  Instead, at
  startup the VM creates an empty disk image in ./<hostname>.qcow2 to
  store the VM's root filesystem.

  Building a VM in this way is efficient because the VM shares its Nix
  store with the host (through a CIFS mount).  However, because the
  Nix store of the host is mounted read-only in the guest, you cannot
  run Nix build actions inside the VM.  Therefore the VM can only be
  reconfigured by re-running "nixos-rebuild build-vm" on the host and
  restarting the VM.

svn path=/nixos/trunk/; revision=16662

modules/virtualisation/qemu-vm.nix

@@ -85,8 +85,12 @@ in
     '';
 
   # Mount the host filesystem via CIFS, and bind-mount the Nix store
-  # of the host into our own filesystem.
-  fileSystems =
+  # of the host into our own filesystem.  We use mkOverride to allow
+  # this module to be applied to "normal" NixOS system configuration,
+  # where the regular value for the `fileSystems' attribute should be
+  # disregarded for the purpose of building a VM test image (since
+  # those filesystems don't exist in the VM).
+  fileSystems = pkgs.lib.mkOverride 50 {}
     [ { mountPoint = "/";
         device = "/dev/vda";
       }