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/device_manager.rs

@@ -4797,7 +4797,7 @@ impl DeviceManager {
 
                     debug!("Closing preserved FDs from virtio-net device: id={id}, fds={fds:?}");
                     for fd in fds {
-                        config.preserved_fds.as_mut().unwrap().retain(|x| *x != fd);
+                        config.preserved_fds.as_mut().unwrap().remove(&fd);
                         // SAFETY: We are closing the only remaining instance of this FD.
                         unsafe {
                             libc::close(fd);