dir storage driver path check is not specific enough

[cluening]

Required information

• Distribution: Fedora
• Distribution version: 39
• The output of "incus info" or if that fails:
  • Kernel version: 6.7.11-200.fc39.x86_64
  • LXC version: lxc-libs-5.0.3-0.2.fc39.x86_64
  • Incus version: incus-6.0.0-0.1.fc39.x86_64
  • Storage backend in use: dir

Issue description

Creating a storage pool using the dir driver whose source is /var/lib/incusfoo/storage-pools/foopool fails with the error:

Error: Source path '/var/lib/incusfoo/storage-pools/foopool' is within the Incus directory

because the location starts with the string /var/lib/incus even though the path itself is different

Steps to reproduce

[root]# mkdir -p /var/lib/incusfoo/storage-pools/foopool
[root]# incus storage create foo dir source=/var/lib/incusfoo/storage-pools/foopool
Error: Source path '/var/lib/incusfoo/storage-pools/foopool' is within the Incus directory

Information to attach

• incusout.txt