vnc: reorganize code for reverse mode

Avoid the dance between csock and vs->lsock.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/qmp.c b/qmp.c
index 36c54c5..31bc3bf 100644
--- a/qmp.c
+++ b/qmp.c
@@ -349,11 +349,9 @@
     }
 }
 
-static void qmp_change_vnc_listen(const char *target, Error **err)
+static void qmp_change_vnc_listen(const char *target, Error **errp)
 {
-    if (vnc_display_open(NULL, target) < 0) {
-        error_set(err, QERR_VNC_SERVER_FAILED, target);
-    }
+    vnc_display_open(NULL, target, errp);
 }
 
 static void qmp_change_vnc(const char *target, bool has_arg, const char *arg,