sasl: Fix authentication when using PLAIN mechanism

sasl: Fix authentication when using PLAIN mechanism

With some authentication mechanism (PLAIN for example), sasl_client_start()
can return SASL_OK, which translates to virNetSASLSessionClientStart()
returning VIR_NET_SASL_COMPLETE.
cyrus-sasl documentation is a bit vague as to what to do in such situation,
but upstream clarified this a bit in
http://asg.andrew.cmu.edu/archive/message.php?mailbox=archive.cyrus-sasl&msg=10104

When we got VIR_NET_SASL_COMPLETE after virNetSASLSessionClientStart() and
if the remote also tells us that authentication is complete, then we should
end the authentication procedure rather than forcing a call to
virNetSASLSessionClientStep(). Without this patch, when trying to use SASL
PLAIN, I get:
error :authentication failed : Failed to step SASL negotiation: -1
(SASL(-1): generic failure: Unable to find a callback: 32775)

This patch is based on a spice-gtk patch by Dietmar Maurer.