xen: flag hvc_xen to be not essential for system boot

The Xen pv console driver is not essential for boot. Set the respective
flag.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211022064800.14978-4-jgross@suse.comSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/tty/hvc/hvc_xen.c

@@ -522,6 +522,7 @@ static struct xenbus_driver xencons_driver = {
 	.remove = xencons_remove,
 	.resume = xencons_resume,
 	.otherend_changed = xencons_backend_changed,
+	.not_essential = true,
 };
 #endif /* CONFIG_HVC_XEN_FRONTEND */