vm: Add support for i686.

Partially fixes <http://bugs.gnu.org/18002>.
Reported by David Thompson <dthompson2@worcester.edu>.

* guix/build/vm.scm (qemu-command): Add optional 'system' parameter.
  Special-case "^i[3456]86$".
* gnu/system/vm.scm (system-qemu-image/shared-store-script): Use it.

gnu/system/vm.scm

@@ -23,6 +23,8 @@
   #:use-module (guix derivations)
   #:use-module (guix packages)
   #:use-module (guix monads)
+  #:use-module ((guix build vm)
+                #:select (qemu-command))
   #:use-module ((gnu packages base)
                 #:select (%final-inputs))
   #:use-module (gnu packages guile)
@@ -414,7 +416,8 @@ OS that shares its store with the host."
           (lambda (port)
             (display
              (string-append "#!" #$bash "/bin/sh
-exec " #$qemu "/bin/qemu-system-x86_64 -enable-kvm -no-reboot -net nic,model=virtio \
+exec " #$qemu "/bin/" #$(qemu-command (%current-system))
+" -enable-kvm -no-reboot -net nic,model=virtio \
   -virtfs local,path=" #$(%store-prefix) ",security_model=none,mount_tag=store \
   -net user \
   -kernel " #$(operating-system-kernel os) "/bzImage \

guix/build/vm.scm

@@ -21,10 +21,12 @@
   #:use-module (guix build linux-initrd)
   #:use-module (guix build install)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 regex)
   #:use-module (ice-9 rdelim)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-26)
-  #:export (load-in-linux-vm
+  #:export (qemu-command
+            load-in-linux-vm
             format-partition
             initialize-root-partition
             initialize-partition-table
@@ -37,12 +39,14 @@
 ;;;
 ;;; Code:
 
-(define (qemu-command)
-  "Return the default name of the QEMU command for the current host."
-  (string-append "qemu-system-"
-                 (substring %host-type 0
-                            (string-index %host-type #\-))))
-
+(define* (qemu-command #:optional (system %host-type))
+  "Return the default name of the QEMU command for SYSTEM."
+  (let ((cpu (substring %host-type 0
+                        (string-index %host-type #\-))))
+    (string-append "qemu-system-"
+                   (if (string-match "^i[3456]86$" cpu)
+                       "i386"
+                       cpu))))
 
 (define* (load-in-linux-vm builder
                            #:key