runtime-rs: introduce a new function generate_vhost_vsock_cid.

Introduce a new function generate_vhost_vsock_cid to generate
a guest CID and set guest CID for vsock fd.
Also this commit wouldn't introduce functional change and it's
just splited from the previous VsockDevice::new().

Fixes: #8474

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

src/runtime-rs/crates/hypervisor/src/device/driver/virtio_vsock.rs

@@ -121,6 +121,21 @@ const CID_RETRY_COUNT: u32 = 50;
 
 impl VsockDevice {
     pub async fn new(id: String) -> Result<Self> {
+        let (guest_cid, vhost_fd) = generate_vhost_vsock_cid()
+            .await
+            .context("generate vhost vsock cid failed")?;
+
+        Ok(Self {
+            id,
+            config: VsockConfig {
+                guest_cid,
+                vhost_fd,
+            },
+        })
+    }
+}
+
+pub async fn generate_vhost_vsock_cid() -> Result<(u32, File)> {
     let vhost_fd = OpenOptions::new()
         .read(true)
         .write(true)
@@ -140,27 +155,19 @@ impl VsockDevice {
         let guest_cid =
             unsafe { vhost_vsock_set_guest_cid(vhost_fd.as_raw_fd(), &(rand_cid as u64)) };
         match guest_cid {
-                Ok(_) => {
+            Ok(_) => return Ok((rand_cid, vhost_fd)),
-                    return Ok(VsockDevice {
-                        id,
-                        config: VsockConfig {
-                            guest_cid: rand_cid,
-                            vhost_fd,
-                        },
-                    });
-                }
             Err(nix::Error::EADDRINUSE) => {
                 // The CID is already in use. Try another one.
+                continue;
             }
             Err(err) => {
                 return Err(err).context("failed to set guest CID");
             }
-            }
+        };
     }
 
     anyhow::bail!(
         "failed to find a free vsock context ID after {} attempts",
         CID_RETRY_COUNT
     );
-    }
 }