iotests: Disable 110 for vmdk.twoGbMaxExtentSparse

The error message for the test case where we have a quorum node for
which no directory name can be generated is different: For
twoGbMaxExtentSparse, it complains that it cannot open the extent file.
For other (sub)formats, it just notes that it cannot determine the
backing file path.  Both are fine, but just disable twoGbMaxExtentSparse
for simplicity's sake.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20190815153638.4600-7-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>