vmbus_server: use a separate max restore version

[SvenGroot]

in #3482, the restore process for VMBus was updated to respect the configured max protocol version and feature flags. However, this is not the behavior we necessarily want in all cases. For example, in OpenHCL we want to be able to allow a feature to be enabled for restore even if otherwise still disabled, so that we can later enable it once we know that all versions we might roll back to can support it.

For this reason, this change makes the version limit during restore operations a separately configurable value.

[Copilot]

Pull request overview

Adds a separate, restore-specific VMBus max protocol/version limit so restore acceptance can be configured independently of the max version/feature flags offered during live negotiation (supporting rollback/servicing scenarios such as OpenHCL).

Changes:

• Introduce max_restore_version on VmbusServerBuilder and plumb it into channels::Server.
• Update restore codepaths to apply the restore-specific max version/feature mask.
• Update OpenVMM wiring and unit tests to use the new restore compatibility setter.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
vm/devices/vmbus/vmbus_server/src/lib.rs	Adds builder option for max_restore_version and applies it during server construction.
vm/devices/vmbus/vmbus_server/src/channels.rs	Adds max_restore_version storage and a setter; updates comments around version limiting.
vm/devices/vmbus/vmbus_server/src/channels/saved_state.rs	Switches restore enforcement from max_version to max_restore_version.
vm/devices/vmbus/vmbus_server/src/channels/tests.rs	Updates restore-related tests to configure restore compatibility directly.
openvmm/openvmm_core/src/worker/dispatch.rs	Wires OpenVMM to set both live and restore max version values from config.