misc: arch/riscv64: streamline #[source] and Error

This streamlines the code base to follow best practices for
error handling in Rust: Each error struct implements
std::error::Error (most due via thiserror::Error derive macro)
and sets its source accordingly.

This allows future work that nicely prints the error chains,
for example.

So far, the convention is that each error prints its
sub error as part of its Display::fmt() impl.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

virtio-devices/src/block.rs

@@ -62,25 +62,25 @@ pub const MINIMUM_BLOCK_QUEUE_SIZE: u16 = 2;
 #[derive(Error, Debug)]
 pub enum Error {
     #[error("Failed to parse the request: {0}")]
-    RequestParsing(block::Error),
+    RequestParsing(#[source] block::Error),
     #[error("Failed to execute the request: {0}")]
-    RequestExecuting(block::ExecuteError),
+    RequestExecuting(#[source] block::ExecuteError),
     #[error("Failed to complete the request: {0}")]
-    RequestCompleting(block::Error),
+    RequestCompleting(#[source] block::Error),
     #[error("Missing the expected entry in the list of requests")]
     MissingEntryRequestList,
     #[error("The asynchronous request returned with failure")]
     AsyncRequestFailure,
     #[error("Failed synchronizing the file: {0}")]
-    Fsync(AsyncIoError),
+    Fsync(#[source] AsyncIoError),
     #[error("Failed adding used index: {0}")]
-    QueueAddUsed(virtio_queue::Error),
+    QueueAddUsed(#[source] virtio_queue::Error),
     #[error("Failed creating an iterator over the queue: {0}")]
-    QueueIterator(virtio_queue::Error),
+    QueueIterator(#[source] virtio_queue::Error),
     #[error("Failed to update request status: {0}")]
-    RequestStatus(GuestMemoryError),
+    RequestStatus(#[source] GuestMemoryError),
     #[error("Failed to enable notification: {0}")]
-    QueueEnableNotification(virtio_queue::Error),
+    QueueEnableNotification(#[source] virtio_queue::Error),
     #[error("Failed to get {lock_type:?} lock for disk image {path}: {error}")]
     LockDiskImage {
         /// The underlying error.