wireguard: Fix wireguard device name to use user-instance

If the user supplies an instance number when creating a wireguard
interface, it should be used in the device name.  If no user-instance
is given, the device instnce (tunnel number) is used.

For example:
    vpp# wireguard create instance 17 listen-port 23023 src 1.2.3.4
            private-key MDEwMjAzMDQwNTA2MDcwODA5MTAxMTEyMTMxNDE1MTY=
    wg17

Type: fix
Fixes: edca1325cf
Signed-off-by: Jon Loeliger <jdl@netgate.com>
Change-Id: Ibf51868bf7b4b9e8a3f85557d05667207873bc91

src/plugins/wireguard/wireguard_if.c

@@ -38,7 +38,8 @@ static u8 *
 format_wg_if_name (u8 * s, va_list * args)
 {
   u32 dev_instance = va_arg (*args, u32);
-  return format (s, "wg%d", dev_instance);
+  wg_if_t *wgi = wg_if_get (dev_instance);
+  return format (s, "wg%d", wgi->user_instance);
 }
 
 u8 *