vmm: disallow ISO rootfs for image versions >= 0.5.0

[h4x3rotab]

Motivation

• The new guest dstack-util setup path no longer accepts or validates a rootfs hash, which combined with the VMM still permitting .iso rootfs attachments allows a host to substitute an unverified rootfs and break the chain-of-trust.
• Provide a minimal, conservative mitigation in the VMM launch path to prevent modern images from being launched with unverified ISO rootfs files while preserving legacy behavior.

Description

• Add an image-version check in vmm/src/app/qemu.rs that reads img_ver = self.image.info.version_tuple() and bail!s when the rootfs extension is "iso" for images with version >= (0, 5, 0) so modern images must use verity-backed rootfs.
• Leave .verity handling unchanged and continue allowing .iso for older/legacy image versions to maintain backward compatibility.
• This is a minimal VMM-side guard that closes the immediate attack vector without reintroducing guest-side hash plumbing.

Testing

• Ran cargo check -p dstack-vmm which completed successfully.

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Codex Task