runtime-rs: Get available port node in the PCIe topology

This commit implements the `find_available_node` function,
which searches the PCIe topology for the first available
`TopologyPortDevice` or `SwitchDownPort`.
If no available node is found in either the `pcie_port_devices`
or the connected switches' downstream ports, the function returns
`None`.

Fixes # 10361

Signed-off-by: alex.lyn <alex.lyn@antgroup.com>

src/runtime-rs/crates/hypervisor/src/device/topology.rs

@@ -318,6 +318,13 @@ pub enum Strategy {
     MultipleRootPorts,
 }
 
+/// Represents an available node in the PCIe topology.
+#[derive(Clone, Debug)]
+pub enum AvailableNode {
+    TopologyPortDevice(TopologyPortDevice),
+    SwitchDownPort(SwitchDownPort),
+}
+
 #[derive(Clone, Debug, Default)]
 pub struct PCIeTopology {
     pub hypervisor_name: String,
@@ -648,6 +655,31 @@ impl PCIeTopology {
 
         Ok(())
     }
+
+    /// Finds an availabled node in the PCIe topology.
+    /// Returns the first available node found, either a TopologyPortDevice or a SwitchDownPort.
+    pub fn find_available_node(&mut self) -> Option<AvailableNode> {
+        // search in pcie_port_devices
+        for port_device in self.pcie_port_devices.values_mut() {
+            if !port_device.allocated {
+                port_device.allocated = true;
+                return Some(AvailableNode::TopologyPortDevice(port_device.clone()));
+            }
+
+            // search in connected switch's downstream ports
+            if let Some(switch) = &mut port_device.connected_switch {
+                for switch_port in switch.switch_ports.values_mut() {
+                    if !switch_port.allocated {
+                        switch_port.allocated = true;
+                        return Some(AvailableNode::SwitchDownPort(switch_port.clone()));
+                    }
+                }
+            }
+        }
+
+        // No available node found
+        None
+    }
 }
 
 // do_add_pcie_endpoint do add a device into PCIe topology with pcie endpoint