bootloader: Adapt vm to new bootloader API.

bootloader: Adapt vm to new bootloader API.

* gnu/build/install.scm (install-boot-config): New procedure.
  (install-grub): Move to (gnu bootloader grub).
* gnu/build/vm.scm (register-bootcfg-root): Rename register-grub.cfg-root and
  adjust accordingly.
  (initialize-hard-disk): Takes a bootloader-package, bootcfg, bootcfg-location and
  bootloader-installer procedure. Adjust accordingly.
* gnu/system/vm.scm (qemu-image): Adjust to initialize-hard-disk.
  (system-disk-image, system-qemu-image, system-qemu-image/shared-store):
  Adjust to qemu-image.