[bug] LXC backups don't work in snapshot mode (bdev specific)

This is because in snapshot mode while running in mfsbdev mode, /dev/loop0 is mounted (as the mfsbdev needs it for `/`).

We can get around it in a number of ways:

* We could disable mfsbdev for rootfs for LXC
* We could find a way to only allow suspend backups for Bdev LXCs