Fix handling of ISO volume size in restricted projects

[stgraber]

When importing an ISO volume into a restricted project, we're currently getting:

stgraber@dakara:~/Downloads$ incus storage volume import --type=iso n-infra:remote win11.iso windows-11
Error: Failed creating custom volume from ISO: Failed checking volume creation allowed: Failed checking if volume creation allowed: Failed getting usage of project entities: Custom volume "windows-11" in project "dev-blue-team" has no "size" config set

Without digging into the code, I suspect that this is simply a small omission in the logic that receives the ISO and writes it into a storage volume. The size of the ISO volume is known precisely ahead of time as unlike other volume types, it's not decompressed or converted in any way on reception, it's just written byte for byte into a block volume.