Don't invoke the auth callback if all credentials were in config file

Don't invoke the auth callback if all credentials were in config file

The remote driver first looks at the libvirt auth config file to
fill in any credentials. It then invokes the auth callback for
any remaining credentials. It was accidentally invoking the
auth callback even if there were not any more credentials
required.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>