wireguard: Fix wireguard device name to use user-instance

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: edca1325cf296bd0f5ff422fc12de2ce7a7bad88
Signed-off-by: Jon Loeliger <jdl@netgate.com>
Change-Id: Ibf51868bf7b4b9e8a3f85557d05667207873bc91