CL skips interfaces that are not listed on getmac output.

Fixes #13606

Change-Id: Ic25c9dc95e8eeff4d84b78e99131a4f97020164c
Reviewed-on: https://go-review.googlesource.com/17994Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>