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") ''