runtime-rs: Add CcwSubChannel to provide devno for CCW devices

To explicitly specify a device number on the QEMU command line
for the following devices using the CCW transport on s390x:

- SerialDevice
- BlockDevice
- VhostUserDevice
- SCSIController
- VSOCKDevice

this commit introduces a new structure CcwSubChannel and implements
the following methods:

- add_device()
- remove_device()
- address_format_ccw()
- set_addr()

You can see the detailed explanation for each method in the comment.

This resolves the 1st part of #10573.

Signed-off-by: Hyounggyu Choi <Hyounggyu.Choi@ibm.com>

src/runtime-rs/crates/hypervisor/src/qemu/cmdline_generator.rs

@@ -369,6 +369,88 @@ impl ToQemuParams for Cpu {
     }
 }
 
+/// Error type for CCW Subchannel operations
+#[derive(Debug)]
+enum CcwError {
+    DeviceAlreadyExists(String), // Error when trying to add an existing device
+    #[allow(dead_code)]
+    DeviceNotFound(String), // Error when trying to remove a nonexistent device
+}
+
+/// Represents a CCW subchannel for managing devices
+#[derive(Debug)]
+struct CcwSubChannel {
+    devices: HashMap<String, u32>, // Maps device IDs to slot indices
+    addr: u32,                     // Subchannel address
+    next_slot: u32,                // Next available slot index
+}
+
+impl CcwSubChannel {
+    fn new() -> Self {
+        Self {
+            devices: HashMap::new(),
+            addr: 0,
+            next_slot: 0,
+        }
+    }
+
+    /// Adds a device to the subchannel.
+    ///
+    /// # Arguments
+    /// - `dev_id`: device ID to add
+    ///
+    /// # Returns
+    /// - `Result<u32, CcwError>`: slot index of the added device
+    ///   or an error if the device already exists
+    fn add_device(&mut self, dev_id: &str) -> Result<u32, CcwError> {
+        if self.devices.contains_key(dev_id) {
+            Err(CcwError::DeviceAlreadyExists(dev_id.to_owned()))
+        } else {
+            let slot = self.next_slot;
+            self.devices.insert(dev_id.to_owned(), slot);
+            self.next_slot += 1;
+            Ok(slot)
+        }
+    }
+
+    /// Removes a device from the subchannel by its ID.
+    ///
+    /// # Arguments
+    /// - `dev_id`: device ID to remove
+    ///
+    /// # Returns
+    /// - `Result<(), CcwError>`: Ok(()) if the device was removed
+    ///  or an error if the device was not found
+    #[allow(dead_code)]
+    fn remove_device(&mut self, dev_id: &str) -> Result<(), CcwError> {
+        if self.devices.remove(dev_id).is_some() {
+            Ok(())
+        } else {
+            Err(CcwError::DeviceNotFound(dev_id.to_owned()))
+        }
+    }
+
+    /// Formats the CCW address for a given slot
+    ///
+    /// # Arguments
+    /// - `slot`: slot index
+    ///
+    /// # Returns
+    /// - `String`: formatted CCW address (e.g. `fe.0.0000`)
+    fn address_format_ccw(&self, slot: u32) -> String {
+        format!("fe.{:x}.{:04x}", self.addr, slot)
+    }
+
+    /// Sets the address of the subchannel.
+    /// # Arguments
+    /// - `addr`: subchannel address to set
+    #[allow(dead_code)]
+    fn set_addr(&mut self, addr: u32) -> &mut Self {
+        self.addr = addr;
+        self
+    }
+}
+
 #[derive(Debug)]
 struct Machine {
     r#type: String,
@@ -1622,10 +1704,15 @@ pub struct QemuCmdLine<'a> {
     knobs: Knobs,
 
     devices: Vec<Box<dyn ToQemuParams>>,
+    ccw_subchannel: Option<CcwSubChannel>,
 }
 
 impl<'a> QemuCmdLine<'a> {
     pub fn new(id: &str, config: &'a HypervisorConfig) -> Result<QemuCmdLine<'a>> {
+        let ccw_subchannel = match bus_type(config) {
+            VirtioBusType::Ccw => Some(CcwSubChannel::new()),
+            _ => None,
+        };
         let mut qemu_cmd_line = QemuCmdLine {
             id: id.to_string(),
             config,
@@ -1637,6 +1724,7 @@ impl<'a> QemuCmdLine<'a> {
             qmp_socket: QmpSocket::new(MonitorProtocol::Qmp)?,
             knobs: Knobs::new(config),
             devices: Vec::new(),
+            ccw_subchannel,
         };
 
         if config.device_info.enable_iommu {