I think the culprit is "Don't count ctx->notifier as progress when polling". It causes aio_poll to do the poll system call before invoking aio_bh_poll(). I followed up in the qemu-devel mailing list.
Fix included in qemu-kvm-rhev-2.12.0-17.el7
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.