Kernel/VirtIO: Ensure proper error propagation in core methods

Simplify core methods in the VirtIO bus handling code by ensuring proper
error propagation. This makes initialization of queues, handling changes
in device configuration, and other core patterns more readable as well.

It also allows us to remove the obnoxious pattern of checking for
boolean "success" and if we get false answer then returning an actual
errno code.

Kernel/Bus/VirtIO/RNG.h

@@ -27,7 +27,7 @@ public:
 private:
     virtual StringView class_name() const override { return "VirtIORNG"sv; }
     explicit RNG(NonnullOwnPtr<TransportEntity>);
-    virtual bool handle_device_config_change() override;
+    virtual ErrorOr<void> handle_device_config_change() override;
     virtual void handle_queue_update(u16 queue_index) override;
     void request_entropy_from_host();