'Callback for the authentication scheme, which will provide username
and password for the login. Reference: ( http://bit.ly/1U5yyQg )
:param  cred: The credentials requested and the return
:type   cred: ``list``
:param  user_data: Custom data provided to the authentication routine
:type   user_data: ``list``
:rtype: ``int``'

def _cred_callback(self, cred, user_data):

    for credential in cred:
        if (credential[0] == libvirt.VIR_CRED_AUTHNAME):
            credential[4] = self._key
        elif (credential[0] == libvirt.VIR_CRED_PASSPHRASE):
            credential[4] = self._secret
    return 0
