From Bug 1513543:
"To allow using blockdev-add for external snapshots (blockdev-snapshot) or blockdev mirror it's necessary to create qcow2 or other formats. Currently it would require a call to qemu-img which is not really a good idea for libvirt since it's hard to manage qemu-img or figure out whether it supports a given format (due to lacking introspection)."
Kevin Wolf introduced x-blockdev-create upstream and is working on turning it into an async job, but it lacks support for creating vmdk images, which is what this BZ is about.
AFAIK we don't have a use-case for supporting VMDK image creation in RHEL/RHV, but it needs to be accepted upstream for completeness. So I'm marking it OtherQA.