IMAGE_FSTYPES vmdk/vdi/qcow2 are special types in image-vm.bbclass: they are equivalent to hdddirect converted to those other formats. With the "image compression" concept extended to "general purpose conversion", it makes more sense to remove those special types (and the corresponding code) and just use conversion commands. The advantage is that these conversion commands then can also be used for other whole-disk image types. Out-dated patch here: https://github.com/pohly/ostro-os/commit/fdb302ca414565d4e84e81a0df07d5c65600ecf9 It was discussed on OE-core and needs further work: http://lists.openembedded.org/pipermail/openembedded-core/2016-July/124517.html
This appears to be resolved by https://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=9d07c736e39a9aa922630b4241eda185a19a11ea