vm: Make the image format a parameter.

* guix/build/vm.scm (load-in-linux-vm): Add #:disk-image-format
  parameter; add 'image-file' variable.  Honor DISK-IMAGE-FORMAT.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Add
  #:disk-image-format parameter, and honor it.
  (qemu-image): Likewise.