arm-autonomy/xenguest-network: add "none" network type to xenguests

This patch adds a new "none" network type that can be set for xenguests
using XENGUEST_IMAGE_NETWORK_TYPE in their configuration.

Issue-Id: SCM-2338
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Change-Id: If4f6de282b0048e5c68d293a3db5b56cd5adce66
Signed-off-by: Ross Burton <ross.burton@arm.com>