IBM i tends to be picky about disks, as well. At least as of a couple years ago, the only way to use local disks with standard 512-byte or 4096-byte sector sizes was through VIOS or similar* storage virtualization mechanisms, so unless QEMU supports 520- or (4096+mumble)-byte virtual SAS disks, you also need to emulate enough of the IBM POWER hypervisor architecture to support running VIOS (IBM's AIX-derived virtual I/O appliance) concurrently in a separate logical partition within the same QEMU VM.

Even if QEMU does support odd-sized virtual sectors, given the integral support for logical partitioning included within modern IBM i releases, various system management and RAS features, etc., I'd be entirely unsurprised to learn that significant firmware support isn't also required to boot the OS at all, regardless of I/O device support.

In which case, you'd either need to reimplement a significant bit of IBM firmware functionality from scratch, or else you'd need to emulate POWER system hardware convincingly enough to fool (possibly hacked) authentic IBM firmware.

Finally, given that AFAIK IBM has never sold unbundled licenses for IBM i (or OS/400), the only potentially legal way to do any of this would be to run QEMU a POWER system already licensed to run IBM i (no technical problem, as all recent POWER systems have excellent Linux support).

*Where, by "similar", I'm thinking of virtual disks used by virtual instances running within an existing IBM i instance, which wouldn't be helpful here.