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>

tests/qemu-iotests/126

@@ -33,6 +33,8 @@ status=1	# failure is the default!
 
 # Needs backing file support
 _supported_fmt qcow qcow2 qed vmdk
+_unsupported_imgopts "subformat=monolithicFlat" \
+                     "subformat=twoGbMaxExtentFlat"
 # This is the default protocol (and we want to test the difference between
 # colons which separate a protocol prefix from the rest and colons which are
 # just part of the filename, so we cannot test protocols which require a prefix)