This turns out to be a bug in qemu 2.12-rc1.
It is fixed by this commit already upstream:
commit 37c51741892a89cf5710f5ac231091fb0a6352c7
Author: Fam Zheng <famz@redhat.com>
Date: Wed Mar 28 00:41:41 2018 +0800
scsi-disk: Don't enlarge min_io_size to max_io_size
Some backends report big max_io_sectors. Making min_io_size the same
value in this case will make it impossible for guest to align memory,
therefore the disk may not be usable at all.
Do not enlarge them when they are zero.
Rich.
--
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