Haven't dealt with live migration, but mounting filesystems should be easy using systemd's unit dependencies.
Just drop a .mount file in /etc/systemd/system and set RequiredBy=systemd-nspawn@something.service and StopWhenUnneeded=true and the filesystem should be mounted before the machine starts and unmounted when the machine is shut down. See the manpages systemd.unit and systemd.mount for details.
Just drop a .mount file in /etc/systemd/system and set RequiredBy=systemd-nspawn@something.service and StopWhenUnneeded=true and the filesystem should be mounted before the machine starts and unmounted when the machine is shut down. See the manpages systemd.unit and systemd.mount for details.