vmm: Use HashSet for VmConfig::preserved_fds

Each VM reboot re-entered the VFIO/virtio-net add paths and re-appended
the same originating fds, leaving duplicates in preserved_fds and a
double-close hazard at final teardown. Switching to HashSet makes
add_preserved_fds idempotent.

Signed-off-by: Bo Chen <bchen@crusoe.ai>
Assisted-by: Claude:Opus-4.7

vmm/src/vm_config.rs

@@ -2,6 +2,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 //
+use std::collections::HashSet;
 use std::net::IpAddr;
 use std::path::{Path, PathBuf};
 #[cfg(feature = "fw_cfg")]
@@ -1065,7 +1066,7 @@ pub struct VmConfig {
     // causes the FDs to be closed early. This allows management software to
     // gracefully clean up resources (e.g., libvirt closes tap devices).
     #[serde(skip)]
-    pub preserved_fds: Option<Vec<i32>>,
+    pub preserved_fds: Option<HashSet<i32>>,
     #[serde(default)]
     pub landlock_enable: bool,
     pub landlock_rules: Option<Box<[LandlockConfig]>>,