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
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