image-fuzzer: Reduce number of generator functions in __init__

Some issues can be found only when a fuzzed image has a partial structure,
e.g. has L1/L2 tables but no refcount ones. Generation of an entirely
defined image limits these cases. Now the Image constructor creates only
a header and a backing file name (if any), other image elements are generated
in the 'create_image' API.

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