qemu-iotests: exclude vmdk for test 042

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>

tests/qemu-iotests/042

@@ -38,7 +38,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.rc
 . ./common.filter
 
-_supported_fmt qcow2 qcow qed vmdk
+_supported_fmt qcow2 qcow qed
 _supported_proto file
 _supported_os Linux