openhcl: remove cmdline to configure shared vis pool + tests

[mattkur]

Tests were setting this flag, but weren't actually testing their device DMA
with shared visibility pool. The shared visibility pool is only used when
the VM is isolated.

Bring this back in one of two ways:

1. Add isolated tests (and get the right behavior), or
2. Change the devices to force into using the shared pool.

Sorta fixes #2185 (the real issue there is that we weren't testing what we thought we were ... :()

[Copilot]

Pull Request Overview

This PR removes the command line configuration option for enabling the shared visibility pool in OpenHCL, along with associated test cases that were using this option. The shared visibility pool is only used when the VM is isolated, so the removed configuration option and tests were not actually testing the intended behavior since they weren't running in an isolated environment.

Key changes:

• Removed the OPENHCL_ENABLE_SHARED_VISIBILITY_POOL environment variable and related configuration logic
• Deleted test cases that were incorrectly attempting to test shared pool behavior in non-isolated environments
• Simplified the shared pool memory allocation logic to only depend on isolation type

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
vmm_tests/vmm_tests/tests/tests/x86_64/openhcl_uefi.rs	Removed nvme_relay_shared_pool test function that was incorrectly testing shared pool behavior
vmm_tests/vmm_tests/tests/tests/x86_64/openhcl_linux_direct.rs	Removed command line options from MANA NIC tests that were not actually enabling shared pool behavior
openhcl/underhill_core/src/worker.rs	Removed enable_shared_visibility_pool field from UnderhillEnvCfg and simplified memory allocation logic
openhcl/underhill_core/src/options.rs	Removed enable_shared_visibility_pool option and related environment variable parsing
openhcl/underhill_core/src/lib.rs	Removed assignment of the removed configuration field

[chris-oo]

Right, now that we have isolated VM tests onboarded, this isn't as useful. We still need to onboard some device tests but those should come soon, i hope.