The implementation comes from Zach's [RFC, PATCH 10/24] i386 Vmi
descriptor changes:

Descriptor and trap table cleanups.  Add cleanly written accessors for
IDT and GDT gates so the subarch may override them.  Note that this
allows the hypervisor to transparently tweak the DPL of the descriptors
as well as the RPL of segments in those descriptors, with no unnecessary
kernel code modification.  It also allows the hypervisor implementation
of the VMI to tweak the gates, allowing for custom exception frames or
extra layers of indirection above the guest fault / IRQ handlers.

Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andi Kleen <ak@suse.de>