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

vmm/src/vm.rs

@@ -142,10 +142,10 @@ pub enum Error {
     PoisonedState,
 
     #[error("Error from device manager: {0:?}")]
-    DeviceManager(DeviceManagerError),
+    DeviceManager(#[source] DeviceManagerError),
 
     #[error("Error initializing VM: {0:?}")]
-    InitializeVm(hypervisor::HypervisorVmError),
+    InitializeVm(#[source] hypervisor::HypervisorVmError),
 
     #[error("No device with id {0:?} to remove")]
     NoDeviceToRemove(String),
@@ -196,7 +196,7 @@ pub enum Error {
     Resume(#[source] MigratableError),
 
     #[error("Memory manager error: {0:?}")]
-    MemoryManager(MemoryManagerError),
+    MemoryManager(#[source] MemoryManagerError),
 
     #[error("Eventfd write error: {0}")]
     EventfdError(#[source] std::io::Error),
@@ -235,16 +235,16 @@ pub enum Error {
     ResizeZone,
 
     #[error("Cannot activate virtio devices: {0:?}")]
-    ActivateVirtioDevices(DeviceManagerError),
+    ActivateVirtioDevices(#[source] DeviceManagerError),
 
     #[error("Error triggering power button: {0:?}")]
-    PowerButton(DeviceManagerError),
+    PowerButton(#[source] DeviceManagerError),
 
     #[error("Kernel lacks PVH header")]
     KernelMissingPvhHeader,
 
     #[error("Failed to allocate firmware RAM: {0:?}")]
-    AllocateFirmwareMemory(MemoryManagerError),
+    AllocateFirmwareMemory(#[source] MemoryManagerError),
 
     #[error("Error manipulating firmware file: {0}")]
     FirmwareFile(#[source] std::io::Error),
@@ -304,7 +304,7 @@ pub enum Error {
     Debug(DebuggableError),
 
     #[error("Error spawning kernel loading thread")]
-    KernelLoadThreadSpawn(std::io::Error),
+    KernelLoadThreadSpawn(#[source] std::io::Error),
 
     #[error("Error joining kernel loading thread")]
     KernelLoadThreadJoin(std::boxed::Box<dyn std::any::Any + std::marker::Send>),
@@ -314,7 +314,7 @@ pub enum Error {
 
     #[cfg(all(target_arch = "x86_64", feature = "guest_debug"))]
     #[error("Error coredumping VM: {0:?}")]
-    Coredump(GuestDebuggableError),
+    Coredump(#[source] GuestDebuggableError),
 
     #[cfg(feature = "igvm")]
     #[error("Cannot open igvm file: {0}")]
@@ -331,10 +331,10 @@ pub enum Error {
     ResumeVm(#[source] hypervisor::HypervisorVmError),
 
     #[error("Error creating console devices")]
-    CreateConsoleDevices(ConsoleDeviceError),
+    CreateConsoleDevices(#[source] ConsoleDeviceError),
 
     #[error("Error locking disk images: Another instance likely holds a lock")]
-    LockingError(DeviceManagerError),
+    LockingError(#[source] DeviceManagerError),
 }
 pub type Result<T> = result::Result<T, Error>;