gnu: guix: Update to 0.4.

* gnu/packages/package-management.scm (guix): Update to 0.4.
  (guix-0.4): Remove.
* gnu/system/vm.scm (qemu-image, system-qemu-image): Use GUIX instead of
  GUIX-0.4.

gnu: guix: Remove warning about 0.4 hack.

* gnu/packages/package-management.scm (guix-0.4): Remove warning.

Merge branch 'master' into core-updates

gnu: guix: Rework 0.4 hack to gracefully handle lack of tarball.

* gnu/packages/package-management.scm (guix-0.4): Fall back to the
  source of GIUX when the tarball doesn't exist.

gnu: vm: Initialize the image's store.

* gnu/system/vm.scm (qemu-image): Add 'initialize-store?' keyword
  parameter.  Use 'guix-register' when INITIALIZE-STORE? is true.
  (system-qemu-image): Pass #:initialize-store? #t.

gnu: guix: Set $localstatedir; add gzip as input.

* gnu/packages/package-management.scm (guix): Add "--localstatedir=/var".
  Add GZIP as an explicit input, to avoid referring to the final gzip.

gnu: Add Guix.

* gnu/packages/package-management.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.