Xen-pv_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 and SMP guest support (NEW!)
* dynamic ticks (NEW!)
* writable pagetables, with late pinning/early unpinning (no shadow pagetable support)
* supports both PAE and non-PAE modes
(non-PAE may be broken at the moment)
* 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
3-19 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.
19-34 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-rc3-git7.

Changes since the previous posting:
- SMP support
- Implemented xen_clockevent, so Xen is fully compatible with CONFIG_NO_HZ and HIGH_RES_TIMERS.
- Finished implementing late pinning/early unpinning of pagetables, which should now be equivalent to xen-unstable in performance.
- Add kmap_atomic_pte for mapping ptes, and therefore support for HIGHPTE
- Fixed crash on systems with >= 4G memory
- Cleanup netfront (Stephen Hemminger)
- Add MAINTAINERS entries
- Fold kill-HAVE_ARCH_MM_LIFETIME.patch into mm-lifetime-hooks.patch
- rebased

Thanks,
J