gnu: vm: Add /etc/shadow in the QEMU image.

* gnu/system/vm.scm (qemu-image): Add 'populate' keyword parameter and
  honor it; make it an input.
  (/etc/shadow): New procedure.
  (example2): Call it; build 'populate' script, and pass it to
  'qemu-image'.

gnu: vm: Remove unused 'qemu-image' argument.

* gnu/system/vm.scm (qemu-image): Remove 'boot-expression' parameter,
  superseded by 'linux-arguments'.

gnu: vm: First stab at building a populated QEMU image.

* gnu/packages/linux-initrd.scm (gnu-system-initrd): New variable.
* gnu/system/vm.scm (qemu-image): Add #:linux-arguments parameter.
  [input->name+derivation]: Add case for 'store-path?' items.
  Remove LOADER from `inputs'.

gnu: vm: 'qemu-image' populates /dev on the target root file system.

* gnu/system/vm.scm (qemu-image): Use (guix build linux-initrd).  Remove
  'mknod' calls; use 'make-essential-device-nodes' to populate /dev on
  the target image.
* gnu/packages/linux-initrd.scm (qemu-initrd): When /root/dev exists,
  don't call 'make-essential-device-nodes'.

gnu: vm: `qemu-image' can copy store closures into the target image.

* gnu/system/vm.scm (qemu-image): Add #:inputs-to-copy and
  #:boot-expression parameters.  Honor them.  Append INPUTS-TO-COPY to
  the #:inputs argument for `expression->derivation-in-linux-vm'.
  (example2): Add #:boot-expression and #:inputs-to-copy arguments.

gnu: `expression->derivation-in-linux-vm' accepts files as inputs.

* gnu/system/vm.scm (expression->derivation-in-linux-vm): Accept items
  in INPUTS that refer to a file instead of a package.

gnu: `expression->derivation-in-linux-vm' export references graphs.

* gnu/system/vm.scm (expression->derivation-in-linux-vm): Add
  #:reference-graphs parameter.  Honor it.  Delete duplicates in
  #:modules argument.

vm: Use more keyword parameters for `expression->derivation-in-linux-vm'.

* gnu/system/vm.scm (expression->derivation-in-linux-vm): Turn `system'
  and `inputs' into keyword parameters.
  (qemu-image, example1): Adjust accordingly.

gnu: Have `qemu-image' explicitly reboot when done.

* gnu/system/vm.scm (qemu-image): Call `reboot' when we're done.  Change
  `menuentry' title.

gnu: Remove QEMU-KVM, which is no longer needed.

* gnu/packages/qemu.scm (qemu-kvm): Remove.
  (qemu): Incorporate parts of QEMU-KVM that used to be inherited.
  (qemu-kvm/smb-shares): Rename to...
  (qemu/smb-shares): ... this.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Adjust
  accordingly.

Add (gnu system vm).

* gnu/system/vm.scm: New file.
* Makefile.am (MODULES): Add it.

Add (gnu system vm).

* gnu/system/vm.scm: New file.
* Makefile.am (MODULES): Add it.