network: add ifindex field for host veth device

network: add ifindex field for host veth device

We should not just record the ifindex for the container's veth device but also
for the host's veth device. This is useful when {configuring,deconfiguring}
veth devices and becomes crucial when calling our lxc-user-nic setuid helper
where we rely on the ifindex to make decisions about whether we are licensed to
perform certain operations on the veth device in question.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>