qemu-iotests: allow qcow2 external discarded clusters to contain stale data

qemu-iotests: allow qcow2 external discarded clusters to contain stale data

Test 244 checks the expected behavior of qcow2 external data files
with respect to zero and discarded clusters.  Filesystems however
are free to ignore discard requests, and this seems to be the
case for overlayfs.  Relax the tests to skip checks on the
external data file for discarded areas, which implies not using
qemu-img compare in the data_file_raw=on case.

This fixes docker tests on RHEL8.

Cc: Kevin Wolf <[email protected]>
Cc: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <20200409191006[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>