KVM: Fix removal of nx capability from guest cpuid

Testing the wrong bit caused kvm not to disable nx on the guest when it is
disabled on the host (an mmu optimization relies on the nx bits being the
same in the guest and host).

This allows Windows to boot when nx is disabled on te host (e.g. when
host pae is disabled).

Signed-off-by: Avi Kivity <avi@qumranet.com>