Add suport for libvirt driver to start lxc from rbd volume.

Currently, the libvirt driver does not support starting LXC from RBD volume. It needs to download the LXC image (normally tens of gigabytes) from Glance, prior to starting it locally, which motivates us to enable LXC to boot from RBD volume to save the long downloading time, and potentially benefit from the much larger bandwidth provided by RBD due to its distributed nature. The idea behind our implementation is to map RBD volume to NOVA node, and then mount it to container_dir before starting a container