KVM: VMX: Fix handling of a fault during NMI unblocked due to IRET

Bit 12 is undefined in any of the following cases:
 If the VM exit sets the valid bit in the IDT-vectoring information field.
 If the VM exit is due to a double fault.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>