vnc: add a more descriptive error message

Currently qemu outputs some low-level error in qemu-sockets.c
when failed to start vnc server.
eg. 'getaddrinfo(127.0.0.1,5902): Name or service not known'

Some libvirt users could not know what's happened with this
unclear error message. This patch added a more descriptive
error message.

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

vl.c

@@ -3584,8 +3584,11 @@ int main(int argc, char **argv, char **envp)
     /* init remote displays */
     if (vnc_display) {
         vnc_display_init(ds);
-        if (vnc_display_open(ds, vnc_display) < 0)
+        if (vnc_display_open(ds, vnc_display) < 0) {
+            fprintf(stderr, "Failed to start VNC server on `%s'\n",
+                    vnc_display);
             exit(1);
+        }
 
         if (show_vnc_port) {
             printf("VNC server running on `%s'\n", vnc_display_local_addr(ds));