qemu-iotests: Add _unsupported_imgopts for vmdk subformats

qemu-iotests: Add _unsupported_imgopts for vmdk subformats

Some cases are not applicable for vmdk subformats those don't support
certain features, e.g. backing file, and some others can't run on
mult-file image, e.g. monolithicFlat. This adds declaration in test
cases to skip them automatically, so that iotests on vmdk can go
more smoothly (without manually picking of cases for each subformat).

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>