dragonball: Fix clippy elided_named_lifetimes

Manually fix `elided_named_lifetimes` clippy warning reported by rust
1.85.1.

```console
error: elided lifetime has a name
   --> src/vm/aarch64.rs:113:10
    |
107 |     fn get_fdt_vm_info<'a>(
    |                        -- lifetime `'a` declared here
...
113 |     ) -> FdtVmInfo {
    |          ^^^^^^^^^ this elided lifetime gets resolved as `'a`
    |
    = note: `-D elided-named-lifetimes` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(elided_named_lifetimes)]`
```

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

src/dragonball/src/vm/aarch64.rs

@@ -110,7 +110,7 @@ impl Vm {
         cmdline: &'a str,
         initrd_config: Option<&'a InitrdConfig>,
         vcpu_manager: &'a MutexGuard<VcpuManager>,
-    ) -> FdtVmInfo {
+    ) -> FdtVmInfo<'a> {
         let guest_memory = vm_memory.memory();
         let vcpu_mpidr = vcpu_manager
             .vcpus()