Fix NMI race with context switch.

Fix NMI race with context switch.

The machine used to auto reboot if an NMI was received in a critical
time window when context switching domains. There is a small time
window when the GDT may become unmapped (after CR3 is updated and
before setting GDTR with the new GDT during a domain context
switch. If an NMI is received during this time window a triple fault
is triggered causing the machine to auto reboot.

Bug found and original patch proposed by Jose Renato Santos
<jsantos@hpl.hp.com>.

Signed-off-by: Keir Fraser <keir@xensource.com>