[libvirt-users] tmpfs mounts for lxc container

I intend to share a single read-only filesystem root among a number of LXC containers. With lxc-tools, I can specify tmpfs bind mounts within the container, for directories that need to be writable, like /var/run and /var/lock. Is there a simple way to do this with libvirt, or do I need to mount the tmpfs in the host first, and specify it as a filesystem in the xml? Cheers, -C-