Zero sized disk is not supported by qemu vmdk driver, exclude vmdk from
the test script.

As tested on vmware-vdiskmanager and vmware workstation, zero sized disk
is not supported by vmware, either.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>