xen: Xen implementation for paravirt_ops

Hi Andi,

This series of patches implements the Xen paravirt-ops interface. It applies to 2.6.21-git13 + patches-2.6.21-git7-070507-1.tar.gz (I think "unwinder" is the only patch which doesn't apply to git13, and the sched-clock patches are the only ones which this series actually needs).

Changes since the last posting:
- More netfront review and cleanup
- Added ability to cleanly halt/reboot guests from outside the domain.
- Folded bugfix patches into their main patch
- Lots of little style and other cleanups

These patches are now moderately well tested, with several successful runs through XenSource's regression test suite, and some amount of non-me testing. While I wouldn't go into production with a xen/paravirt_ops kernel right now, it does seem pretty functional.

This series generally restricts itself to Xen-specific parts of the tree, though it does make a few small changes elsewhere.

It includes:
- some helper routines for allocating address space and walking pagetables
- Xen interface header files
- Core Xen implementation (boot, mmu, events, time)
- Efficient late-pinning/early-unpinning pagetable handling
- Virtualized time, including stolen time
- SMP support
- Preemption support
- Batched pagetable updates
- Xen console, based on hvc console
- Xenbus
- Netfront, the paravirtualized network device
- Blockfront, the paravirtualized block device

Thanks,
J