KVM: VMX: include all IRQ window exits in statistics

irq_window_exits only tracks IRQ window exits due to user space
requests, nmi_window_exits include all exits. The latter makes more
sense, so let's adjust irq_window_exits accounting.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>