vm: Rewrite support procedures to use gexps.

* gnu/system/vm.scm (%imported-modules): Remove.
  (expression->derivation-in-linux-vm): Remove 'inputs' parameter.
  Rename 'imported-modules' to 'modules'.  Rewrite using gexps and
  'gexp->derivation'.
  (qemu-image): Add 'qemu' parameter.  Pass NAME to
  'expression->derivation-in-linux-vm'.  Rewrite using gexps.  Remove
  #:inputs argument to 'expression->derivation-in-linux-vm'.
  (operating-system-default-contents): Rewrite using gexps.
* gnu/system.scm (operating-system-profile-derivation): Rename to...
  (operating-system-profile): ... this.  Adjust callers.
  (operating-system-profile-directory): Remove.