In openQA, we install Tumbleweed in textmode on a single hdd.

Then, extra_tests_filesystem boots this system with an additional disk, but
installed system is now /dev/sdb instead of /dev/sda.

See: https://openqa.opensuse.org/tests/1083171#step/lvm/6

qemu options seem to be fine:
-device virtio-scsi-pci,id=scsi0 -blockdev
driver=file,node-name=hd0-overlay0-file,filename=/var/lib/openqa/pool/2/raid/hd0-overlay0,cache.no-flush=on
-blockdev
driver=qcow2,node-name=hd0-overlay0,file=hd0-overlay0-file,cache.no-flush=on
-device
virtio-blk-device,id=hd0-device,drive=hd0-overlay0,bootindex=0,serial=hd0
-blockdev
driver=file,node-name=hd1-file,filename=/var/lib/openqa/pool/2/raid/hd1,cache.no-flush=on
-blockdev driver=qcow2,node-name=hd1,file=hd1-file,cache.no-flush=on -device
virtio-blk-device,id=hd1-device,drive=hd1,serial=hd1

with hd0 the installed system:
running /usr/bin/qemu-img create -f qcow2 -b
/var/lib/openqa/pool/2/opensuse-Tumbleweed-aarch64-20191109-textmode@aarch64.qcow2
/var/lib/openqa/pool/2/raid/hd0-overlay0 32212254720
Formatting '/var/lib/openqa/pool/2/raid/hd0-overlay0', fmt=qcow2
size=32212254720
backing_file=/var/lib/openqa/pool/2/opensuse-Tumbleweed-aarch64-20191109-textmode@aarch64.qcow2
cluster_size=65536 lazy_refcounts=off refcount_bits=16


So, not sure where the problem could come from.
Maybe, the UEFI firmware could change the order?