Create QCOW2 image rather than raw for writable partitions.

Bug: 190503448
Test: vm create-partition /data/local/tmp/blah 1000000
Change-Id: Iff9269d71ea3e5dcff5fbfd1f5a62abfa4633bc0
diff --git a/vm/src/main.rs b/vm/src/main.rs
index e2c11a8..2c93ec4 100644
--- a/vm/src/main.rs
+++ b/vm/src/main.rs
@@ -104,6 +104,7 @@
 ) -> Result<(), Error> {
     let image = OpenOptions::new()
         .create_new(true)
+        .read(true)
         .write(true)
         .open(image_path)
         .with_context(|| format!("Failed to create {:?}", image_path))?;