qcow2 image corrupted after snapshot+bitmap action
Description of problem:
When taking a backup of the VM via snapshot + bitmap, the qcow2 image became corrupt:
`qcow2: Marking image as corrupt: Preventing invalid write on metadata (overlaps with bitmap directory); further corruption events will be suppressed`

This resulted in a corrupt (unfix-able) image (see #2909).

While this process is something that happens multiple times a day, we never hit any issue.
The underlying storage didn't report any error, so it seems like something inside qemu broke the image.
Steps to reproduce:
Unfortunately, I was unable to reproduce this issue yet.
Additional information: