vnc: JPEG should be disabled if the client don't set tight quality

Disable JPEG compression by default and only enable it if the
VNC client has sent the requested quality.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

vnc.c

diff --git a/vnc.c b/vnc.c
index ed0e096..9cf38d1 100644
--- a/vnc.c
+++ b/vnc.c
@@ -1644,7 +1644,7 @@
     vs->features = 0;
     vs->vnc_encoding = 0;
     vs->tight_compression = 9;
-    vs->tight_quality = 9;
+    vs->tight_quality = -1; /* Lossless by default */
     vs->absolute = -1;
 
     /*