gexp: Allow use of high-level objects in #:references-graphs.

* guix/gexp.scm (lower-reference-graphs): New procedure.
  (gexp->derivation)[graphs-file-names]: New procedure.
  Use 'lower-reference-graphs', and augment #:inputs argument as a
  function of #:references-graphs.
* doc/guix.texi (G-Expressions): Adjust 'gexp->derivation' documentation
  accordingly.
* tests/gexp.scm ("gexp->derivation, store copy"): Remove reference to
  TWO in BUILD-DRV.  Use TWO directly in #:references-graphs argument.
  ("gexp->derivation #:references-graphs"): New test.
* gnu/system/vm.scm (qemu-image): Remove variable 'graph'; use INPUTS as
  the #:references-graphs argument to
  'expression->derivation-in-linux-vm'.