fix(guest): disable the ZFS pool root mountpoint

[kvinwang]

Problem

A newly created ZFS pool inherits the default root-dataset mountpoint. This can mount the dstack pool root separately from the explicitly managed dstack/data dataset, creating an unintended mount in the guest filesystem.

Fix

Create the pool with -m none, leaving the pool root unmounted. Only dstack/data, whose mountpoint is explicitly configured by the guest setup code, is mounted.

Implementation

• fix(guest): disable the ZFS pool root mountpoint

Changed path:

• dstack/dstack-util/src/system_setup.rs

Scope

This PR is based directly on master and contains only the ZFS root-mountpoint change.

Verification

• cargo test -p dstack-util --no-run: passed
• git diff --check origin/master...HEAD: passed

[Copilot]

Pull request overview

This PR updates the guest storage initialization logic to ensure the ZFS pool root dataset is never mounted, preventing unintended mounts in the guest filesystem and leaving only the explicitly managed dstack/data dataset mounted at the configured mount point.

Changes:

• Create the ZFS pool with zpool create ... -m none so the pool root dataset has mountpoint=none.
• Keep creating dstack/data with an explicit mountpoint (-o mountpoint=$mount_point) as the sole mounted dataset.

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