libvirt: Support IPv6 with LXC

Libvirt's LXC implementation exposes a read-only  `/proc/sys/net` to the
guests. This causes some of the guest's default network configuration scripts
to fail.

This patch works-around the issue by using `post-up` hooks to configure IPv6.

Closes-Bug: 1340791

Change-Id: I805cad98d855fcb2c90b07e98ad3653d1620bd42
Merge "libvirt: Support IPv6 with LXC"