xen: Xen implementation for paravirt_ops

Hi Andi,

This series of patches implements the Xen paravirt-ops interface.
It applies to 2.6.21-rc7 + your patches + the last batch of pv_ops
patches I posted.

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

These patches include:
 - some helper routines for allocating address space and walking pagetables
 - Xen interface header files
 - Core Xen implementation
 - 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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