I was going to upgrade Ovirt from 3.1 to 3.2, and in prepration, I started to export VMs. I thought that I had enough space on export -domain, but it seems that the thinly provisioned disks will use all the space they've been assigned. So I needed to abort exporting.
Exporting locks the images, so I was a little bit afraid that aborting export with good old 'kill' would leave them into that state. After finding http://firstname.lastname@example.org/msg08588.html, I decided to bit the bullet and just kill the processes. Ovirt actually worked very well, and all images became unlocked automatically. Hooray for proper error handling.
Try at your own risk, though.