vnc: really call zlib if we want zlib

send_framebuffer_update() was calling hextile instead of zlib
since commit 70a4568fe0c5a64adaa3da5030b7109e5199e692.

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 774a0b3..8db1557 100644
--- a/vnc.c
+++ b/vnc.c
@@ -661,7 +661,7 @@
 {
     switch(vs->vnc_encoding) {
         case VNC_ENCODING_ZLIB:
-            vnc_hextile_send_framebuffer_update(vs, x, y, w, h);
+            vnc_zlib_send_framebuffer_update(vs, x, y, w, h);
             break;
         case VNC_ENCODING_HEXTILE:
             vnc_framebuffer_update(vs, x, y, w, h, VNC_ENCODING_HEXTILE);