xen: use a better chardev type check

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

hw/xen/xen-common.c

@@ -38,7 +38,7 @@ static int store_dev_info(int domid, Chardev *cs, const char *string)
     int ret = -1;
 
     /* Only continue if we're talking to a pty. */
-    if (strncmp(cs->filename, "pty:", 4)) {
+    if (!CHARDEV_IS_PTY(cs)) {
         return 0;
     }
     pts = cs->filename + 4;