vnc-stubs: Allow -vnc none

Currently `-vnc none` is fatal when built with `--disable-vnc`.  Make
vnc_parse accept "none", so QEMU still run without using vnc.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Message-id: 20201009014032.3507-1-jandryuk@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/vnc-stubs.c

@@ -12,6 +12,9 @@ int vnc_display_pw_expire(const char *id, time_t expires)
 };
 QemuOpts *vnc_parse(const char *str, Error **errp)
 {
+    if (strcmp(str, "none") == 0) {
+        return NULL;
+    }
     error_setg(errp, "VNC support is disabled");
     return NULL;
 }