instance.img is actually added as a writable partition

Bootloader doesn't use the partition yet.

Bug: 193504400
Test: atest MicrodroidHostTestCases
Change-Id: I7138d966630ebf79c9b7cbc2ea7343852460e2ec
diff --git a/virtualizationservice/src/payload.rs b/virtualizationservice/src/payload.rs
index 28ef502..a176e71 100644
--- a/virtualizationservice/src/payload.rs
+++ b/virtualizationservice/src/payload.rs
@@ -162,6 +162,7 @@
     temporary_directory: &Path,
     apk_file: File,
     idsig_file: File,
+    instance_file: File,
     mut apexes: Vec<ApexConfig>,
     vm_config: &mut VirtualMachineRawConfig,
 ) -> Result<()> {
@@ -189,5 +190,12 @@
         });
     }
 
+    // instance image is at the second partition in the second disk.
+    vm_config.disks[1].partitions.push(Partition {
+        label: "vm-instance".to_owned(),
+        images: vec![ParcelFileDescriptor::new(instance_file)],
+        writable: true,
+    });
+
     Ok(())
 }