hash_type - One of: LIBSSH2_HOSTKEY_HASH_MD5 or
LIBSSH2_HOSTKEY_HASH_SHA1.
Returns the computed digest of the remote system's hostkey. The length of
the returned string is hash_type specific (e.g. 16 bytes for MD5,
20 bytes for SHA1).
RETURN VALUE
Computed hostkey hash value. or NULL if the session has not yet been started
up. (The hash consists of raw binary bytes, not hex digits, so is not
directly printable.)