linux-initrd: Make the virtio kernel modules optional.

* gnu/system/linux-initrd.scm (qemu-initrd): Add #:virtio? parameter.
  Use it.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Pass
  'qemu-initrd' #:virtio?.
  (virtualized-operating-system): Likewise.

gnu/system/linux-initrd.scm

@@ -203,16 +203,18 @@ initrd code."
 
 (define* (qemu-initrd file-systems
                       #:key
-                      guile-modules-in-chroot?
                       (qemu-networking? #t)
-                      volatile-root?)
+                      virtio?
+                      volatile-root?
+                      guile-modules-in-chroot?)
   "Return a monadic derivation that builds an initrd for use in a QEMU guest
 where the store is shared with the host.  FILE-SYSTEMS is a list of
 file-systems to be mounted by the initrd, possibly in addition to the root
 file system specified on the kernel command line via '--root'.
 
 When QEMU-NETWORKING? is true, set up networking with the standard QEMU
-parameters.
+parameters.  When VIRTIO? is true, load additional modules so the initrd can
+be used as a QEMU guest with para-virtualized I/O drivers.
 
 When VOLATILE-ROOT? is true, the root file system is writable but any changes
 to it are lost.
@@ -221,6 +223,11 @@ When GUILE-MODULES-IN-CHROOT? is true, make core Guile modules available in
 the new root.  This is necessary is the file specified as '--load' needs
 access to these modules (which is the case if it wants to even just print an
 exception and backtrace!)."
+  (define virtio-modules
+    ;; Modules for Linux para-virtualized devices, for use in QEMU guests.
+    '("virtio.ko" "virtio_ring.ko" "virtio_pci.ko"
+      "virtio_balloon.ko" "virtio_blk.ko" "virtio_net.ko"))
+
   (define cifs-modules
     ;; Modules needed to mount CIFS file systems.
     '("md4.ko" "ecb.ko" "cifs.ko"))
@@ -235,8 +242,9 @@ exception and backtrace!)."
 
   (define linux-modules
     ;; Modules added to the initrd and loaded from the initrd.
-    `("virtio.ko" "virtio_ring.ko" "virtio_pci.ko"
-      "virtio_balloon.ko" "virtio_blk.ko" "virtio_net.ko"
+    `(,@(if (or virtio? qemu-networking?)
+            virtio-modules
+            '())
       ,@(if (find (file-system-type-predicate "cifs") file-systems)
             cifs-modules
             '())

gnu/system/vm.scm

@@ -154,6 +154,7 @@ made available under the /xchg CIFS share."
        (initrd       (if initrd                   ; use the default initrd?
                          (return initrd)
                          (qemu-initrd %linux-vm-file-systems
+                                      #:virtio? #t
                                       #:guile-modules-in-chroot? #t))))
 
     (define builder
@@ -349,7 +350,9 @@ of the GNU system as described by OS."
   "Return an operating system based on OS suitable for use in a virtualized
 environment with the store shared with the host."
   (operating-system (inherit os)
-    (initrd (cut qemu-initrd <> #:volatile-root? #t))
+    (initrd (cut qemu-initrd <>
+                 #:volatile-root? #t
+                 #:virtio? #t))
     (file-systems (cons* (file-system
                            (mount-point "/")
                            (device "/dev/vda1")