dragonball: Fix clippy repr_packed_without_abi

Fix `repr_packed_without_abi` clippy warning as suggested by rust
1.85.1.

```console
error: item uses `packed` representation without ABI-qualification
   --> dbs_pci/src/msi.rs:468:1
    |
466 |   #[repr(packed)]
    |          ------ `packed` representation set here
467 |   #[derive(Clone, Copy, Default, PartialEq)]
468 | / pub struct MsiState {
469 | |     msg_ctl: u16,
470 | |     msg_addr_lo: u32,
471 | |     msg_addr_hi: u32,
472 | |     msg_data: u16,
473 | |     mask_bits: u32,
474 | | }
    | |_^
    |
    = warning: unqualified `#[repr(packed)]` defaults to `#[repr(Rust, packed)]`, which has no stable ABI
    = help: qualify the desired ABI explicity via `#[repr(C, packed)]` or `#[repr(Rust, packed)]`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#repr_packed_without_abi
    = note: `-D clippy::repr-packed-without-abi` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::repr_packed_without_abi)]`
```

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

src/dragonball/dbs_acpi/src/rsdp.rs

@@ -5,7 +5,7 @@
 // RSDP (Root System Description Pointer) is a data structure used in the ACPI programming interface.
 use vm_memory::ByteValued;
 
-#[repr(packed)]
+#[repr(Rust, packed)]
 #[derive(Clone, Copy, Default)]
 pub struct Rsdp {
     pub signature: [u8; 8],

src/dragonball/dbs_acpi/src/sdt.rs

@@ -2,7 +2,7 @@
 // Copyright (c) 2023 Alibaba Cloud
 //
 // SPDX-License-Identifier: Apache-2.0
-#[repr(packed)]
+#[repr(Rust, packed)]
 pub struct GenericAddress {
     pub address_space_id: u8,
     pub register_bit_width: u8,

src/dragonball/dbs_pci/src/msi.rs

@@ -463,7 +463,7 @@ impl PciCapability for MsiCap {
 }
 
 /// Struct to manage PCI Message Signalled Interrupt controller working state.
-#[repr(packed)]
+#[repr(Rust, packed)]
 #[derive(Clone, Copy, Default, PartialEq)]
 pub struct MsiState {
     msg_ctl: u16,

src/dragonball/dbs_pci/src/msix.rs

@@ -32,7 +32,7 @@ pub const MSIX_TABLE_ENTRIES_MODULO: u64 = 16;
 /// This struct is the shadow copy of the PCI MSI-x capability. Guest device drivers read from/write
 /// to this struct. There's another struct MsixState, which maintains the working state about the
 /// PCI MSI-x controller.
-#[repr(packed)]
+#[repr(Rust, packed)]
 #[derive(Clone, Copy, Default, PartialEq)]
 pub struct MsixCap {
     // Capability ID