Use fs-verity digests proto to build the directory

The proto contains path -> digest mapping. In this change, we start to
use the provided protobuf to build the directory. Digest is not yet
used.

Bug: 206869687
Test: atest AuthFsHostTest ComposHostTestCases MicrodroidTestCase
Change-Id: I0f03219c1243f20f9ec5099a451c6f588add9608
diff --git a/vm/src/main.rs b/vm/src/main.rs
index a466a4c..ad8c201 100644
--- a/vm/src/main.rs
+++ b/vm/src/main.rs
@@ -86,7 +86,7 @@
         cpu_affinity: Option<String>,
 
         /// Paths to extra idsig files.
-        #[structopt(long)]
+        #[structopt(long = "extra-idsig")]
         extra_idsigs: Vec<PathBuf>,
     },
     /// Run a virtual machine