Support multi-file partitions for composite disk

This is a follow-up to 1fee0d8da6bb440127861e278a7dad09b7f506fd.

Bug: 190503456
Test: MicrodroidHostTestCases
Change-Id: If43bf3d10fc773877bba8b02a3088cd26a814ef9
diff --git a/virtualizationservice/aidl/android/system/virtualizationservice/Partition.aidl b/virtualizationservice/aidl/android/system/virtualizationservice/Partition.aidl
index 825c3da..9b8658b 100644
--- a/virtualizationservice/aidl/android/system/virtualizationservice/Partition.aidl
+++ b/virtualizationservice/aidl/android/system/virtualizationservice/Partition.aidl
@@ -20,8 +20,8 @@
     /** A label for the partition. */
     @utf8InCpp String label;
 
-    /** The backing file descriptor of the partition image. */
-    ParcelFileDescriptor image;
+    /** The backing file descriptors of the partition images. */
+    ParcelFileDescriptor[] images;
 
     /** Whether the partition should be writable by the VM. */
     boolean writable;
diff --git a/virtualizationservice/src/composite.rs b/virtualizationservice/src/composite.rs
index 5f792fd..7b5a258 100644
--- a/virtualizationservice/src/composite.rs
+++ b/virtualizationservice/src/composite.rs
@@ -336,9 +336,9 @@
     Ok((composite_image, files))
 }
 
-/// Given the AIDL config containing a list of partitions, with a [`ParcelFileDescriptor`] for each
-/// partition, return the list of file descriptors which must be passed to the mk_cdisk child
-/// process and the composite disk image partition configuration for it.
+/// Given the AIDL config containing a list of partitions, with [`ParcelFileDescriptor`]s for each
+/// partition, return the list of file descriptors which must be passed to the composite disk image
+/// partition configuration for it.
 fn convert_partitions(partitions: &[Partition]) -> Result<(Vec<PartitionInfo>, Vec<File>), Error> {
     // File descriptors to pass to child process.
     let mut files = vec![];
@@ -346,24 +346,27 @@
     let partitions = partitions
         .iter()
         .map(|partition| {
-            // TODO(b/187187765): This shouldn't be an Option.
-            let file = partition
-                .image
-                .as_ref()
-                .context("Invalid partition image file descriptor")?
-                .as_ref()
-                .try_clone()
-                .context("Failed to clone partition image file descriptor")?;
-            let size = get_partition_size(&file)?;
-            let fd = file.as_raw_fd();
-            files.push(file);
+            let image_files = partition
+                .images
+                .iter()
+                .map(|image| {
+                    let file = image
+                        .as_ref()
+                        .try_clone()
+                        .context("Failed to clone partition image file descriptor")?;
+
+                    let size = get_partition_size(&file)?;
+                    let fd = file.as_raw_fd();
+                    let partition_info_file =
+                        PartitionFileInfo { path: format!("/proc/self/fd/{}", fd).into(), size };
+                    files.push(file);
+                    Ok(partition_info_file)
+                })
+                .collect::<Result<Vec<_>, Error>>()?;
 
             Ok(PartitionInfo {
                 label: partition.label.to_owned(),
-                files: vec![PartitionFileInfo {
-                    path: format!("/proc/self/fd/{}", fd).into(),
-                    size,
-                }],
+                files: image_files,
                 partition_type: ImagePartitionType::LinuxFilesystem,
                 writable: partition.writable,
             })