qemu-iotests: Skip 099 for VMDK subformats with desc file

VMDK extent parsing code doesn't handle the JSON file name, so the case
fails for these subformats. Disabled them.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1417571370-19495-1-git-send-email-famz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>