image-fuzzer: Add generators of L1/L2 tables

Entries in L1/L2 entries are based on a portion of random guest clusters.
L2 entries contain offsets to host image clusters filled with random data.
Clusters for L1/L2 tables and guest data are selected randomly.

Signed-off-by: Maria Kustova <maria.k@catit.be>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
