devices: ivshmem: Don't remap file on BAR reprogramming

The ivshmem device was remapping the host file every time the BAR was
reprogrammed. This occurs on the vCPU thread and so was falling foul of
our improved seccomp filters. Avoid this by maintaining the existing
region when the device BARs are changed and just adjust the guest GPA.

Assisted-by: Codex:GPT-5.6
Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/src/device_manager.rs

@@ -4676,7 +4676,6 @@ impl DeviceManager {
             devices::IvshmemDevice::new(
                 id.clone(),
                 ivshmem_cfg.size as u64,
-                Some(ivshmem_cfg.path.clone()),
                 ivshmem_ops.clone(),
                 snapshot,
             )
@@ -5601,6 +5600,16 @@ impl IvshmemOps for IvshmemHandler {
             false,
         )
         .map_err(|e| IvshmemError::CreateUserMemoryRegion(e.into()))?;
+        let region = Arc::new(region);
+        let mapping = self.create_userspace_mapping(start_addr, region.clone())?;
+        Ok((region, mapping))
+    }
+
+    fn create_userspace_mapping(
+        &mut self,
+        start_addr: u64,
+        region: Arc<MmapRegion<AtomicBitmap>>,
+    ) -> Result<UserspaceMapping, IvshmemError> {
         let mem_slot = {
             let mut manager = self.memory_manager.lock().unwrap();
             // SAFETY: guaranteed by MmapRegion invariants
@@ -5616,14 +5625,12 @@ impl IvshmemOps for IvshmemHandler {
             }
         }
         .map_err(|e| IvshmemError::CreateUserspaceMapping(e.into()))?;
-        let region = Arc::new(region);
-        let mapping = UserspaceMapping {
-            mapping: region.clone(),
+        Ok(UserspaceMapping {
+            mapping: region,
             mem_slot,
             addr: GuestAddress(start_addr),
             mergeable: false,
-        };
-        Ok((region, mapping))
+        })
     }
 
     fn unmap_ram_region(&mut self, mapping: UserspaceMapping) -> Result<(), IvshmemError> {