Think you have to map the rootfs after changing /etc/pve/lxc/
LXD once had a tool for that "fuidshift" you have to find a way to get that, I thought it was a static binary.
Only thing i can think about (untested):
Create container with CentOS 6 -> stop it -> extract data to the rootfs of that container -> start it.
Its worth a try I think.
Good you didn't delete your data, i thought nothing was mounted......
What gives:
zfs list -o name,mounted,mountpoint
You can manualy mount them one by mount with "zfs mount <mountpoint>"