vm: 'iso9660-image' produces a single-file output.

* gnu/system/vm.scm (expression->derivation-in-linux-vm): Add
  #:single-file-output? and pass it to 'load-in-linux-vm'.
(iso9660-image): Pass #:single-file-output? to
'expression->derivation-in-linux-vm'.
* gnu/build/vm.scm (load-in-linux-vm): Add #:single-file-output? and
honor it.

gnu/build/vm.scm

@@ -76,11 +76,14 @@
                            (qemu (qemu-command)) (memory-size 512)
                            linux initrd
                            make-disk-image?
+                           single-file-output?
                            (disk-image-size (* 100 (expt 2 20)))
                            (disk-image-format "qcow2")
                            (references-graphs '()))
   "Run BUILDER, a Scheme file, into a VM running LINUX with INITRD, and copy
-the result to OUTPUT.
+the result to OUTPUT.  If SINGLE-FILE-OUTPUT? is true, copy a single file from
+/xchg to OUTPUT.  Otherwise, copy the contents of /xchg to a new directory
+OUTPUT.
 
 When MAKE-DISK-IMAGE? is true, OUTPUT will contain a VM image of
 DISK-IMAGE-SIZE bytes resulting from the execution of BUILDER, which may
@@ -137,8 +140,17 @@ the #:references-graphs parameter of 'derivation'."
 
   ;; When MAKE-DISK-IMAGE? is true, the image is in OUTPUT already.
   (unless make-disk-image?
-    (mkdir output)
-    (copy-recursively "xchg" output)))
+    (if single-file-output?
+        (let ((graph? (lambda (name stat)
+                        (member (basename name) references-graphs))))
+          (match (find-files "xchg" (negate graph?))
+            ((result)
+             (copy-file result output))
+            (x
+             (error "did not find a single result file" x))))
+        (begin
+          (mkdir output)
+          (copy-recursively "xchg" output)))))
 
 
 ;;;
@@ -356,7 +368,7 @@ SYSTEM-DIRECTORY is the name of the directory of the 'system' derivation."
 (define* (make-iso9660-image grub config-file os-drv target
                              #:key (volume-id "GuixSD_image") (volume-uuid #f))
   "Given a GRUB package, creates an iso image as TARGET, using CONFIG-FILE as
-Grub configuration and OS-DRV as the stuff in it."
+GRUB configuration and OS-DRV as the stuff in it."
   (let ((grub-mkrescue (string-append grub "/bin/grub-mkrescue")))
     (mkdir-p "/tmp/root/var/run")
     (mkdir-p "/tmp/root/run")

gnu/system/vm.scm

@@ -105,16 +105,19 @@
                                              (guile-for-build
                                               (%guile-for-build))
 
+                                             (single-file-output? #f)
                                              (make-disk-image? #f)
                                              (references-graphs #f)
                                              (memory-size 256)
                                              (disk-image-format "qcow2")
                                              (disk-image-size 'guess))
   "Evaluate EXP in a QEMU virtual machine running LINUX with INITRD (a
-derivation).  In the virtual machine, EXP has access to all its inputs from the
-store; it should put its output files in the `/xchg' directory, which is
-copied to the derivation's output when the VM terminates.  The virtual machine
-runs with MEMORY-SIZE MiB of memory.
+derivation).  The virtual machine runs with MEMORY-SIZE MiB of memory.  In the
+virtual machine, EXP has access to all its inputs from the store; it should
+put its output file(s) in the '/xchg' directory.
+
+If SINGLE-FILE-OUTPUT? is true, copy a single file from '/xchg' to OUTPUT.
+Otherwise, copy the contents of /xchg to a new directory OUTPUT.
 
 When MAKE-DISK-IMAGE? is true, then create a QEMU disk image of type
 DISK-IMAGE-FORMAT (e.g., 'qcow2' or 'raw'), of DISK-IMAGE-SIZE bytes and
@@ -164,6 +167,7 @@ made available under the /xchg CIFS share."
                                 #:linux linux #:initrd initrd
                                 #:memory-size #$memory-size
                                 #:make-disk-image? #$make-disk-image?
+                                #:single-file-output? #$single-file-output?
                                 #:disk-image-format #$disk-image-format
                                 #:disk-image-size size
                                 #:references-graphs graphs)))))
@@ -219,6 +223,7 @@ INPUTS is a list of inputs (as for packages)."
            (reboot))))
    #:system system
    #:make-disk-image? #f
+   #:single-file-output? #t
    #:references-graphs inputs))
 
 (define* (qemu-image #:key