There is a bug in libvirt-python-1.2.1 that makes the keyboard do nothing when booting a LXC. It's been solved in git: http://libvirt.org/git/?p=libvirt-python.git;a=commit;h=ea9339c878ffc344a3ef0cc659b7bce87125678f I applied that patch and my two test containers work fine. Without the patch they don't work.
With version 1.2.9 being the oldest version in tree this bug should be fixed by now.