Setting up guix ports needs to be done in Geiser REPL synchronously,
otherwise this operation may not be finished before the further
evaluating of guile code.

* emacs/guix-guile.el (guix-guile-prompt?): New function.
* emacs/guix-geiser.el (guix-geiser-eval-in-repl-synchronously): New
  function.
* emacs/guix-devel.el (guix-devel-setup-repl): Use it.