libvirt users - Feb 2016 - High i/o-wait in guest but no i/o-wait on host

Hi,
I have an issue with one of our CentOS 7 hypervisors. The system is a
Dell server equipped with Samsung 840 Pro SSDs.
What happens is that the guest does about 40MB/s of writes (mostly MySQL
inserts) and ends up becoming almost unusable because of high i/o-wait
numbers yet when I check on the host vmstat consistently shows i/o-wait
being 0 the whole time.
This is weird for two reasons:
a) I would not expect 40MB/s to create such an extreme congestion on the
SSDs.
b) If that congestion is in fact real I would expect to see non-zero
i/o-wait numbers on the host created by the corresponding qemu process.

Right now it looks like the i/o requests get stuck in the guest even
though there is no congestion on the host. Could this be a virtio bug?
Has anyone an explanation for this strange behavior?

Regards,
  Dennis