misc: vsock: 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/vsock/csm/mod.rs

@@ -33,10 +33,10 @@ pub enum Error {
     TxBufFull,
     /// An I/O error occurred, when attempting to flush the connection TX buffer.
     #[error("Error flushing TX buffer: {0}")]
-    TxBufFlush(std::io::Error),
+    TxBufFlush(#[source] std::io::Error),
     /// An I/O error occurred, when attempting to write data to the host-side stream.
     #[error("Error writing to host side stream: {0}")]
-    StreamWrite(std::io::Error),
+    StreamWrite(#[source] std::io::Error),
 }
 
 type Result<T> = std::result::Result<T, Error>;