iotests: Disable 126 for flat vmdk subformats

iotest 126 requires backing file support, which flat vmdks cannot offer.
Skip this test for such subformats.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190815153638.4600-8-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>