vmm: Error out on migration & snapshot if on-demand restoring

If there is an active on-demand restoration then reject any requests to
migrate or snapshot this VM as the memory will not be available for the
snapshot.

Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/src/vm.rs

@@ -201,6 +201,9 @@ pub enum Error {
     #[error("VM is currently migrating and can't be modified")]
     VmMigrating,
 
+    #[error("VM on-demand memory restore is still in progress")]
+    VmRestoring,
+
     #[error("Cannot clone EventFd")]
     EventFdClone(#[source] io::Error),
 
@@ -3041,6 +3044,10 @@ impl Vm {
         self.memory_manager.lock().unwrap().guest_memory()
     }
 
+    pub fn restoring(&self) -> bool {
+        self.memory_manager.lock().unwrap().restoring()
+    }
+
     pub fn device_tree(&self) -> Arc<Mutex<DeviceTree>> {
         self.device_manager.lock().unwrap().device_tree()
     }