Ubuntu's libvirt currently ships with its own Linux Containers
implementation (confusingly called libvirt-lxc).
This code has nothing to do with upstream LXC, confuses users who expect
that it does and is in general a much worse experience that the one we
provide through LXD in Ubuntu.
We'd rather not have to maintain that feature in main and given that
libvirt backends can't be split into separate packages (at least
trivially), we're now looking at turning libvirt-lxc altogether.
Red Hat, who was the main driver for this particular driver has also dropped
support for it back in March 2015 (in favor of Docker support).
If you are a current user of libvirt-lxc, we'd be very interested to
hear from you about your use case for it, especially about anything
which libvirt-lxc is providing which you can't achieve with LXC/LXD.