if=ide lands on piix IDE on the i440fx machine (which UEFI guests boot
on since the vioif fix); minimized illumos images carry no ata/cmdk
driver, so the CIDATA seed silently disappears and first-boot
provisioning is skipped. An explicit AHCI controller needs only the
ahci driver on every machine type, and still keeps the seed off the
root disk virtio ordering.
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