KVM: MMU: emulated cmpxchg8b should be atomic on i386

Emulate cmpxchg8b atomically on i386. This is required to avoid a guest
pte walker from seeing a splitted write.

[avi: make it compile]

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>