ovmm_entry: Remove need for unsafe in tests

[smalis-msft]

This file really shouldn't need unsafe, we can just mock out the environment variable instead of actually modifying it.

[Copilot]

Pull request overview

Removes the need for unsafe in openvmm_entry CLI parsing tests by refactoring the autocache disk-spec parsing so tests can inject a cache-path value instead of mutating process-wide environment variables.

Changes:

• Added DiskCliKind::parse_autocache(...) helper to parse autocache:[key]:<kind> given a cache path input.
• Updated DiskCliKind’s FromStr implementation to use the new helper when parsing autocache.
• Reworked tests to avoid std::env::set_var/remove_var and added coverage for key/no-key and missing-cache-path cases.