intg: Return strings from ctypes wrappers
    
    Python module ctypes directly uses C functions from libraries.
    C functions usually returns "char *" when string is expected.
    But python3 uses unicode for string. Encoding returned bytes
    ("char *") to unicode strings simplify tests in python3
    
    Reviewed-by: Martin Basti <mbasti@redhat.com>
    (cherry picked from commit fd2dfed53be8e3a0b63a1b268be34c2dc11ce837)