dev, virtio: Use of Unix socket for virtIO 9P device

dev, virtio: Use of Unix socket for virtIO 9P device

This commit adds support for diod to use a unix socket, rather
than a TCP port.  We don't rely on the IP-based connection as we
directly use pipes to interact with diod.  This allows it to work
on any system, even if the specific port is taken by another diod
instance (or similar).  Secondly, the Unix socket could in theory
be used to debug.  However, this functionality has not been
tested.

Change-Id: I616e0ad8768da1dfc867de3af98cdfbb22a72d63
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2820
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>