iotests: qcow2 COW with minimal L2 cache size

This adds a test case to test 103 for performing a COW operation in a
qcow2 image using an L2 cache with minimal size (which should be at
least two clusters so the COW can access both source and destination
simultaneously).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>