Volume sub-paths are not counted by Incus

[p-revo]

Required information

• Distribution: Debian Bookworm
• Distribution version: 12
• The output of "incus info": incus-info.txt
  • Kernel version: 6.1.0-23-amd64
  • LXC version: 6.0.1
  • Incus version: 6.4
  • Storage backend in use: btrfs

Issue description

The incus storage volume list <pool> command outputs wrong numbers in the volume USED-BY column, if any sub-paths of a volume are attached to a container.
As a consequence, inucs also allows to delete a storage volume, while it is still in use by a container
The issue is also mentioned here

Steps to reproduce

$ incus launch images:debian/bookworm deb1
$ incus launch images:debian/bookworm deb2
$ incus storage volume create default demovol
$ incus config device add deb1 demovol disk pool=default source=demovol path=/mnt/demovol
$ incus shell deb1
root@deb1:~# cd /mnt/demovol
root@deb1:/mnt/demovol# mkdir testdir
root@deb1:/mnt/demovol# echo "test" > testdir/testfile
root@deb1:/mnt/demovol# exit
$ incus config device add deb2 demovol disk pool=default source=demovol/testdir path=/mnt/demovolsub
$ incus storage volume list default

The list command now indicates, that the volume tempvol is used by 1 container, while its is actually used by 2 containers (deb1 and deb2). If deb1 now gets removed, the storage volume tempvol can be deleted, even if it is still used by deb2.