iotests: Make 137 work with data_file

iotests: Make 137 work with data_file

When using an external data file, there are no refcounts for data
clusters.  We thus have to adjust the corruption test in this patch to
not be based around a data cluster allocation, but the L2 table
allocation (L2 tables are still refcounted with external data files).

Furthermore, we should not print qcow2.py's list of incompatible
features because it differs depending on whether there is an external
data file or not.

With those two changes, the test will work both with and without
external data files (once that options works with the iotests at all).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-20-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>