Adds the required constants and new SockaddrVM type for working
with Linux VM sockets.  For more information, see:
https://vmsplice.net/~stefan/stefanha-kvm-forum-2015.pdf.

Fixes golang/go#19434

Change-Id: I9aefb35441b2c7dc412bdfb806ebe08d5c20f6b1
Reviewed-on: https://go-review.googlesource.com/37937
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>