rust: migration: implement ToMigrationState as part of impl_vmstate_bitsized

This is most likely desirable, and is the easiest way to migrate
a bit-sized value without peeking at the innards of the bilge crate.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/migration/src/vmstate.rs

@@ -296,6 +296,25 @@ macro_rules! impl_vmstate_bitsized {
                                           as ::bilge::prelude::Number>::UnderlyingType
                                          as $crate::vmstate::VMState>::VARRAY_FLAG;
         }
+
+        impl $crate::migratable::ToMigrationState for $type {
+            type Migrated = <<$type as ::bilge::prelude::Bitsized>::ArbitraryInt
+                                          as ::bilge::prelude::Number>::UnderlyingType;
+
+            fn snapshot_migration_state(&self, target: &mut Self::Migrated) -> Result<(), $crate::InvalidError> {
+                *target = Self::Migrated::from(*self);
+                Ok(())
+            }
+
+            fn restore_migrated_state_mut(
+                &mut self,
+                source: Self::Migrated,
+                version_id: u8,
+            ) -> Result<(), $crate::InvalidError> {
+                *self = Self::from(source);
+                Ok(())
+            }
+        }
     };
 }