KVM: nVMX: Avoid reading VM_EXIT_INTR_ERROR_CODE needlessly on nested exits

We only need to update vm_exit_intr_error_code if there is a valid exit
interruption information and it comes with a valid error code.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>