vnc: call sasl_server_init() only when required

vnc: call sasl_server_init() only when required

VNC server is calling sasl_server_init() during startup of QEMU, even
if SASL auth has not been enabled.

This may create undesirable warnings like "Could not find keytab file:
/etc/qemu/krb5.tab" when the user didn't configure SASL on host and
started VNC server.

Instead, only initialize SASL when needed. Note that HMP/QMP "change
vnc" calls vnc_display_open() again, which will initialize SASL if
needed.

Fix assignment in if condition, while touching this code.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1609327

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20180907063634.359-1-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>