Provide a custom Validate for OpInstanceMultiAlloc

This is needed, as we're not strictly separating validation from
populating the defaults. We do this all in one function. If we now
receive a non complete OpInstanceCreate opcode in instances, we would
run into AttributeErrors on those.

So we run the validation + recursively on all instances opcodes.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>