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
3-15 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.
16-26 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-rc2-git1.

Changes since the previous posting:
- Moved the arch_*_mmap stubs to asm-generic/mm_hooks.h, and included that in asm-*/mmu_context.h (James Bottomley)
- Use unsigned long consistently for addresses (Andi Kleen)
- Moved xenboot_console declaration to its own header. (Andi Kleen)
- Renamed FIX_PARAVIRT to FIX_PARAVIRT_BOOTMAP to emphasize that its intended for bootstrap only (Ingo Molnar)
- Cleaned up headers a fair bit. Moved a lot of the native_* functions to their "home" headers, so that they can be used either for native paravirt boots, or when CONFIG_PARAVIRT is disabled. This cuts down on duplication, and makes paravirt.h simpler.
- Various cleanups from Adrian Bunk

Thanks,
J