linux-initrd: Add USB kernel modules to the default initrd.

* gnu/system/linux-initrd.scm (base-initrd)[linux-modules]: Add
  usb-storage, uas, usbkbd, and usbhid.
* gnu/system/vm.scm (system-disk-image): Remove #:extra-modules argument
  to 'base-initrd'.

gnu/system/linux-initrd.scm

@@ -184,8 +184,10 @@ (define (file-system-type-predicate type)
 
   (define linux-modules
     ;; Modules added to the initrd and loaded from the initrd.
-    `("ahci"                                   ;for SATA controllers
-      "pata_acpi" "pata_atiixp"                ;for ATA controllers
+    `("ahci"                                  ;for SATA controllers
+      "pata_acpi" "pata_atiixp"               ;for ATA controllers
+      "usb-storage" "uas"                     ;for the installation image etc.
+      "usbkbd" "usbhid"                       ;USB keyboards, for debugging
       ,@(if (or virtio? qemu-networking?)
             virtio-modules
             '())

gnu/system/vm.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -278,12 +278,11 @@ (define file-systems-to-keep
 
   (let ((os (operating-system (inherit os)
               ;; Since this is meant to be used on real hardware, don't
-              ;; install QEMU networking or anything like that, but make sure
-              ;; USB mass storage devices are available.
+              ;; install QEMU networking or anything like that.  Assume USB
+              ;; mass storage devices (usb-storage.ko) are available.
               (initrd (lambda (file-systems . rest)
                         (apply base-initrd file-systems
                                #:volatile-root? #t
-                               #:extra-modules '("usb-storage.ko")
                                rest)))
 
               ;; Force our own root file system.