qemu-img should generate VMDK with an EOS marker when `has_marker` flag enabled
Additional information:
I generate a empty volume with capacity 1G and try to deploy it as a part of OVF. This would fail. 

But when I append an EOS marker to that VMDK, which is actually a zeroed sector, the deployed procedure succeeded.

This case merely happened if VMDK has data, since `qemu-img` always write at least one grain(64 KB). So the padding part will be recognized as  EOS marker.

I have written a temporary patch for this and it works fine for me. I'm glad to send it for review.