vm: Make root file system type a parameter, and default to ext4.

* gnu/system/vm.scm (qemu-image): Add #:file-system-type parameter.
  Pass it to 'initialize-hard-disk'.
* guix/build/linux-initrd.scm (mount-root-file-system): Always honor
  TYPE.
  (boot-system): Change #:root-fs-type to default to "ext4".  Update
  docstring.
* guix/build/vm.scm (initialize-hard-disk): Remove #:mkfs parameter; add
  #:file-system-type.  Adjust 'mkfs' invocation and 'mount' call to
  honor #:file-system-type.