Create instance.img

The file is a per-VM persistent storage which will contain identity
information like the certs of the payload.

As the first step, this change creates instance.img upon creation of a
VM and passes it to virtualizationservice. For now, the file is not
passed down to crosvm yet.

Bug: 193504400
Test: atest MicrodroidHostTestCases
Change-Id: I44115a93abdd23c36a4c52a38e5355bfa3bf69b3
diff --git a/vmconfig/src/lib.rs b/vmconfig/src/lib.rs
index 1a16d30..4a5b3b1 100644
--- a/vmconfig/src/lib.rs
+++ b/vmconfig/src/lib.rs
@@ -153,7 +153,7 @@
 }
 
 /// Try to open the given file and wrap it in a [`ParcelFileDescriptor`].
-fn open_parcel_file(filename: &Path, writable: bool) -> Result<ParcelFileDescriptor> {
+pub fn open_parcel_file(filename: &Path, writable: bool) -> Result<ParcelFileDescriptor> {
     Ok(ParcelFileDescriptor::new(
         OpenOptions::new()
             .read(true)