KVM: VMX: Add 'else' to split mutually exclusive case

Each if branch in handle_external_interrupt_irqoff() is mutually
exclusive. Add 'else' to make it clear and also avoid some unnecessary
check.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>