Rename payload metadata partition

/dev/block/by-name/metadata is used by others as well.

Renamed it as payload-metadata.

Bug: 191097666
Test: MicrodroidHostTestCases
Change-Id: I873ee2acbe38772e59c96b3904023ac0d1573621
diff --git a/virtualizationservice/src/payload.rs b/virtualizationservice/src/payload.rs
index 18e7de5..76c55de 100644
--- a/virtualizationservice/src/payload.rs
+++ b/virtualizationservice/src/payload.rs
@@ -158,7 +158,7 @@
 
     // put metadata at the first partition
     let mut partitions = vec![Partition {
-        label: "metadata".to_owned(),
+        label: "payload-metadata".to_owned(),
         paths: vec![metadata_path],
         writable: false,
     }];