Move kernelExtraConfig to common-config.nix (for x86)

It's bad to have the kernel config scattered across two places.  (This
should also be done for the other architectures.)

Also, restore Xen and KVM guest support in Linux 3.10.

pkgs/os-specific/linux/kernel/common-config.nix

@@ -265,7 +265,22 @@ with stdenv.lib;
   ''}
 
   # Virtualisation.
+  PARAVIRT y
+  ${if versionAtLeast version "3.10" then ''
+    HYPERVISOR_GUEST y
+  '' else ''
+    PARAVIRT_GUEST y
+  ''}
+  KVM_GUEST y
+  ${optionalString (versionOlder version "3.7") ''
+    KVM_CLOCK y
+  ''}
+  XEN y
   XEN_DOM0? y
+  KSM y
+  ${optionalString (!stdenv.is64bit) ''
+    HIGHMEM64G? y # We need 64 GB (PAE) support for Xen guest support.
+  ''}
 
   # Media support.
   ${optionalString (versionAtLeast version "3.6") ''

pkgs/top-level/platforms.nix

@@ -10,19 +10,6 @@ rec {
     # Currently ignored - it should be set according to 'system' once it is
     # not ignored. This is for stdenv-updates.
     kernelArch = "i386";
-    kernelExtraConfig =
-      ''
-        # Virtualisation (KVM, Xen...).
-        HYPERVISOR_GUEST? y #3.10 version of the paravirt options
-        PARAVIRT_GUEST? y #Doesn't exist in 3.10
-        KVM_CLOCK? y #Part of KVM_GUEST since linux 3.7
-        KVM_GUEST? y #Doesn't exist in 3.10
-        XEN? y #Doesn't exist in 3.10
-        KSM y
-
-        # We need 64 GB (PAE) support for Xen guest support.
-        HIGHMEM64G? y
-      '';
   };
 
   pc_simplekernel = pc // {