Qcow2 image stuck as locked after host crash

After a host crash, the qcow2 image of the VM, stored on a remote NFS share, has become inaccessible. Libvirt/QEMU reports that 'failed to get "write" lock\nIs another process using the image [/path/nfs/image.qcow2]?'. No process is accessing the image from either host or the network share side. There is no obvious way in qemu-img to force unlocking the file or repair the image (attempting a qemu-img check with -r all results in qemu-img complaining about the lock and being unable to do force-share=on on anything but readonly images).

I'm currently attempting to fix this by converting the image via 'qemu-img convert -U -f qcow2 -O qcow2 image.qcow2 image_2.gcow2', though this will likely take some time.

Using QEMU 3.1.0