vnc: fix vnc client authentication

Commit 800567a61 updated the code to the generic crypto API
and mixed up encrypt and decrypt functions in
procotol_client_auth_vnc.
(Used to be: deskey(key, EN0) which encrypts, and was
changed to qcrypto_cipher_decrypt in 800567a61.)
Changed it to qcrypto_cipher_encrypt now.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
