Xen PVH support

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
I just commited Xen PVH support to -current.
The PVH mode was introduced by Xen as a way to take advantage of hardware
virtualisation features, without the cost of HVM and its qemu process
in the dom0.
Xen PVH uses only PV drivers for I/O, in the same way the old PV mode does.
But, unlike PV, PVH uses native CPU and page table management operations.

Because PVH is much closer to native hardware, the PVH support is in
GENERIC and not in the XEN3_DOMU kernel.

Note that on amd64, SVS is enabled by default and can be quite slow if
your hardware doens't have the EPT extension. I've not tested it with
a EPT-capable CPU yet.
SVS can be disabled at boot time with '-3' on the kernel boot arguments
(extra="-3" in the config file).

To use PVH, add
type = "pvh"
to your domU config file, and change the kernel= line to point to
a GENERIC kernel.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--