iotests: Test copy offloading with external data file

iotests: Test copy offloading with external data file

This adds a test for 'qemu-img convert' with copy offloading where the
target image has an external data file. If the test hosts supports it,
it tests both the case where copy offloading is supported and the case
where it isn't (otherwise we just test unsupported twice).

More specifically, the case with unsupported copy offloading tests
qcow2_alloc_cluster_abort() with external data files.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200211094900.17315-4-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>