Oh, the ubuntu: remote uses images I’m not used to. Their rootfs is at /dev/sda1 with cloudinit-rootfs LABEL. Anyway, here’s what I did to get the VM booting the -virtual/-generic kernel:
lxc shell $NAME
apt-get update && apt-get install linux-image-virtual
apt-get autopurge linux-image-kvm
rm /etc/default/grub.d/40-force-partuuid.cfg # disable initrdless booting
update-grub
reboot