Computes the HMAC for a binary data of length.
 This is a convenience wrapper for Hmac, 
            get_string and g_hmac_unref.
The hexadecimal string returned will be in lower case.
| key | the key to use in the HMAC | 
| data | binary blob to compute the HMAC of | 
| digest_type | a ChecksumType to use for the HMAC | 
| key_len | the length of the key | 
| length | length of  | 
| the HMAC of the binary data as a string in hexadecimal. The returned string should be freed with g_free when done using it. |