devices: add debug-console device

This commit adds the debug-console (or debugcon) device to CHV. It is a
very simple device on I/O port 0xe9 supported by QEMU and BOCHS. It is
meant for printing information as easy as possible, without any
necessary configuration from the guest at all.

It is primarily interesting to OS/kernel and firmware developers as they
can produce output as soon as the guest starts without any configuration
of a serial device or similar. Furthermore, a kernel hacker might use
this device for information of type B whereas information of type A are
printed to the serial device.

This device is not used by default by Linux, Windows, or any other
"real" OS, but only by toy kernels and during firmware development.

In the CLI, it can be configured similar to --console or --serial with
the --debug-console parameter.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

vmm/src/vm_config.rs

@@ -396,6 +396,27 @@ pub fn default_consoleconfig_file() -> Option<PathBuf> {
     None
 }
 
+#[cfg(target_arch = "x86_64")]
+#[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
+pub struct DebugConsoleConfig {
+    #[serde(default)]
+    pub file: Option<PathBuf>,
+    pub mode: ConsoleOutputMode,
+    /// Optionally dedicated I/O-port, if the default port should not be used.
+    pub iobase: Option<u16>,
+}
+
+#[cfg(target_arch = "x86_64")]
+impl Default for DebugConsoleConfig {
+    fn default() -> Self {
+        Self {
+            file: None,
+            mode: ConsoleOutputMode::Off,
+            iobase: Some(devices::debug_console::DEFAULT_PORT as u16),
+        }
+    }
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 pub struct DeviceConfig {
     pub path: PathBuf,
@@ -537,6 +558,9 @@ pub struct VmConfig {
     pub serial: ConsoleConfig,
     #[serde(default = "default_console")]
     pub console: ConsoleConfig,
+    #[cfg(target_arch = "x86_64")]
+    #[serde(default)]
+    pub debug_console: DebugConsoleConfig,
     pub devices: Option<Vec<DeviceConfig>>,
     pub user_devices: Option<Vec<UserDeviceConfig>>,
     pub vdpa: Option<Vec<VdpaConfig>>,