=== DF-4503 run on guest (load-half smoke test) ===
$ ./trigger_DF4503 /dev/vbd0s1d 2
done: 13341 reads on /dev/vbd0s1d
$
RESULT: load generator works, but the bug path is NOT reachable on this
guest: no ahci(4) controller (PCI list: PIIX3 IDE + virtio only), and
QEMU does not emulate SATA port multipliers or FBS at all, so the
co-active-command window (AP_F_FBSS_ENABLED + PM register command timing
out while a co-issued standard command is still on-chip) cannot be
created. Compile + load-half run verified only.