vmm: make landlock configs VMM-level config

This requires stashing the config values in `struct Vmm`. The configs
should be validated before before creating the VMM thread. Refactor the
code and update documentation where necessary.

The place where the rules are applied remain the same.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

fuzz/fuzz_targets/http_api.rs

@@ -190,8 +190,6 @@ impl RequestHandler for StubApiRequestHandler {
                 platform: None,
                 tpm: None,
                 preserved_fds: None,
-                landlock_enable: false,
-                landlock_config: None,
             })),
             state: VmState::Running,
             memory_actual_size: 0,