It seems you are not passing the private key to libssh2.
SSH pubkey authentication requires two machines, a server and client. The client should generate a private and public keypair, where the public key should be copied to the server using any method available prior to pubkey authentication attempts.The private key should never leave the client.
When connecting to the server using the client, one should use the private key. Authenticating using only the public key only will not work, as you can only encrypt the negotiation packets, but never actually decrypt them. Reading through the documentation also states that the private key is a must, while the public key might be set to NULL in some cases.
Your public key looks fine. I believe the error you are getting might be related to the private key which you set to NULL. Try passing your private key to libssh2, and make sure to add your public key to the servers authorized keys.