I had the exact same problem on Debian 9 with custom Linux 5 kernels that I compiled myself.
I recompiled the latest stable 5.0.6 kernel following instructions here (although I was also unable to find all exact options).
Still doesn’t work.
Following @HulaHoop’s instructions I edited out the
<blkiotune>
<weight>250</weight>
</blkiotune>
In the VM settings. Works fine now.
So probably a bug/regression in the new kernel?