The first is a F18 disk image, of virtual size 20 GB.
The second is a completely blank disk image, of virtual size 1 GB.
Note that you will need qemu from git (or possibly 1.5)
for any of this to work.
Unfortunately supporting VDHX format is not as easy as I expected. You can't simply look at specific offsets in the file to determine the VDHX version, capacity or backing file. You have to actually do quite a bit of careful parsing of headers. This will require some re-factoring of the way libvirt probes storage metadata to support, rather than just adding a few offsets to the existing data tables.
Closing accordingly to v2v bug 761199. If there is real requirement for this feature please reopen this bug with additional justification. Thanks.