cache_req: add api to create ldb_result from message
    
    Some sysdb methods doesn't return ldb_result as output but return
    ldb_message instead. Changing sysdb to be consistent is too big
    so I added this helper function that will wrap resulting message
    into ldb_result.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>