gnu: linux-initrd: Make the pseudo-tty device nodes.

* guix/build/linux-initrd.scm (make-essential-device-nodes): Create
  /dev/ptmx and /dev/pts.
* gnu/system/vm.scm (qemu-image): Umount /fs/dev/pts before /fs.

gnu/system/vm.scm

@@ -383,7 +383,13 @@ such as /etc files."
                              (system* grub "--no-floppy"
                                       "--boot-directory" "/fs/boot"
                                       "/dev/sda"))
-                            (zero? (system* umount "/fs"))
+                            (begin
+                              (when (file-exists? "/fs/dev/pts")
+                                ;; Unmount devpts so /fs itself can be
+                                ;; unmounted (failing to do that leads to
+                                ;; EBUSY.)
+                                (system* umount "/fs/dev/pts"))
+                              (zero? (system* umount "/fs")))
                             (reboot))))))))
     #:system system
     #:inputs `(("parted" ,parted)

guix/build/linux-initrd.scm

@@ -114,6 +114,14 @@
                   (device-number 4 n))
            (loop (+ 1 n)))))
 
+  ;; Pseudo ttys.
+  (mknod (scope "dev/ptmx") 'char-special #o666
+         (device-number 5 2))
+
+  (unless (file-exists? (scope "dev/pts"))
+    (mkdir (scope "dev/pts")))
+  (mount "none" (scope "dev/pts") "devpts")
+
   ;; Rendez-vous point for syslogd.
   (mknod (scope "dev/log") 'socket #o666 0)
   (mknod (scope "dev/kmsg") 'char-special #o600 (device-number 1 11))