SASL may prompt for credentials after either a 'start' or 'step'
invocation. In both cases the code to handle this is the same.
Refactor this code into a separate method to reduce the duplication,
since the complexity is about to grow

* src/remote/remote_driver.c: Refactor interaction with SASL
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>