vnc: support password expire

This patch adds support for expiring passwords to vnc.  It adds a new
vnc_display_pw_expire() function which specifies the time when the
password will expire.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/ui/vnc.h b/ui/vnc.h
index 9619b24..4f895be 100644
--- a/ui/vnc.h
+++ b/ui/vnc.h
@@ -120,6 +120,7 @@
 
     char *display;
     char *password;
+    time_t expires;
     int auth;
     bool lossy;
 #ifdef CONFIG_VNC_TLS