xend: Add support for XCP Windows PV drivers

xend: Add support for XCP Windows PV drivers

This patch adds support for XCP Windows paravirtual drivers to run on
Xen. The drivers are currently provided in binary-only format from
Citrix. At a minimum, this patch is useful for performance comparisons
vs GPLPV drivers.

Live migration and save/resume are functional but set the guest clock
to the 1970's. The clock must be manually adjusted for the guest's ntp
to resume accurate timekeeping.

Before rebooting windows at the end of driver installation create the
registry key
HKLM\System\CurrentControlSet\Services\xenevtchn\Parameters. Add to it
a DWORD called SetFlags with a value of 0x10000000.

Signed-off-by: Keith Coleman <keith@scaltro.com>