Xen-paravirt_ops: Xen guest implementation for paravirt_ops interface

Hi Andi,

This patch series implements the Linux Xen guest as a paravirt_ops
backend.  The features in implemented this patch series are:
 * domU only
 * UP only (most code is SMP-safe, but there's no way to create a new vcpu)
 * writable pagetables, with late pinning/early unpinning
   (no shadow pagetable support)
 * supports both PAE and non-PAE modes
 * xen hvc console (console=hvc0)
 * virtual block device (blockfront)
 * virtual network device (netfront)

The patch series is in three parts:
  1- 2	Cleanup patches to various parts of the kernel
  3-14	Extensions to the core code and/or paravirt ops, needed to support
	Xen. Includes hooks into get/setting pte values and the new
	patching machinery.
 15-26	The Xen paravirt_ops implementation itself.

(Some of the earlier patches in the series have already been posted,
 but are included to make the series self-contained.)

I've tried to make each patch as self-explanatory as possible.  The
series is based on 2.6.21-rc1.

Changes since the previous posting:
- implemented new patching scheme, which should make the overhead of
  running on native approach zero, and Xen overhead a fair bit lower
- rebased
- some small review comment updates
- trailing whitespace stripped
- accurate diffstats

Thanks,
	J
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