services: hurd-vm: Disable password-based authentication for root.

With offloading to a childhurd is enabled, allowing password-less root
login in the childhurd to anyone amounts to providing write access to
the host’s store to anyone.  Thus, disable password-based root logins in
the childhurd.

* gnu/services/virtualization.scm (%hurd-vm-operating-system): Change
‘permit-root-login’ to 'prohibit-password.
* gnu/tests/virtualization.scm (%childhurd-os): Provide a custom ‘os’
field for ‘hurd-vm-configuration’.
* doc/guix.texi (Virtualization Services): Remove mention of
password-less root login.

doc/guix.texi

@@ -35722,11 +35722,6 @@ guix shell tigervnc-client -- vncviewer localhost:5900
 The default configuration (see @code{hurd-vm-configuration} below)
 spawns a secure shell (SSH) server in your GNU/Hurd system, which QEMU
 (the virtual machine emulator) redirects to port 10222 on the host.
-Thus, you can connect over SSH to the childhurd with:
-
-@example
-ssh root@@localhost -p 10022
-@end example
 
 The childhurd is volatile and stateless: it starts with a fresh root
 file system every time you restart it.  By default though, all the files

gnu/services/virtualization.scm

@@ -1080,7 +1080,7 @@ that will be listening to receive secret keys on port 1004, TCP."
                         (openssh-configuration
                          (openssh openssh-sans-x)
                          (use-pam? #f)
-                         (permit-root-login #t)
+                         (permit-root-login 'prohibit-password)
                          (allow-empty-passwords? #t)
                          (password-authentication? #t)))
 

gnu/tests/virtualization.scm

@@ -31,6 +31,7 @@
   #:use-module (gnu services)
   #:use-module (gnu services dbus)
   #:use-module (gnu services networking)
+  #:use-module (gnu services ssh)
   #:use-module (gnu services virtualization)
   #:use-module (gnu packages ssh)
   #:use-module (gnu packages virtualization)
@@ -228,7 +229,19 @@
 (define %childhurd-os
   (simple-operating-system
    (service dhcp-client-service-type)
-   (service hurd-vm-service-type)))
+   (service hurd-vm-service-type
+            (hurd-vm-configuration
+             ;; Allow root login with an empty password to simplify the test
+             ;; below.
+             (os (operating-system
+                   (inherit %hurd-vm-operating-system)
+                   (services
+                    (modify-services (operating-system-user-services
+                                      %hurd-vm-operating-system)
+                      (openssh-service-type
+                       config => (openssh-configuration
+                                  (inherit config)
+                                  (permit-root-login #t)))))))))))
 
 (define (run-childhurd-test)
   (define os