misc: virtio-devices: 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/console.rs

@@ -52,15 +52,15 @@ enum Error {
     #[error("Descriptor chain too short")]
     DescriptorChainTooShort,
     #[error("Failed to read from guest memory: {0}")]
-    GuestMemoryRead(vm_memory::guest_memory::Error),
+    GuestMemoryRead(#[source] vm_memory::guest_memory::Error),
     #[error("Failed to write to guest memory: {0}")]
-    GuestMemoryWrite(vm_memory::guest_memory::Error),
+    GuestMemoryWrite(#[source] vm_memory::guest_memory::Error),
     #[error("Failed to write_all output: {0}")]
-    OutputWriteAll(io::Error),
+    OutputWriteAll(#[source] io::Error),
     #[error("Failed to flush output: {0}")]
-    OutputFlush(io::Error),
+    OutputFlush(#[source] io::Error),
     #[error("Failed to add used index: {0}")]
-    QueueAddUsed(virtio_queue::Error),
+    QueueAddUsed(#[source] virtio_queue::Error),
 }
 
 #[derive(Copy, Clone, Debug, Serialize, Deserialize)]