[virtio] Mark virtio_drivers::Hal implementation as unsafe

As required by virtio_drivers v0.4.0.

Bug: 274732281
Test: m pvmfw_img && atest vmbase_example.integration_test
Change-Id: Ie30fae6962bbe6af2e711b9bb7c475df4a9f9c7b
diff --git a/vmbase/example/src/pci.rs b/vmbase/example/src/pci.rs
index 117cbc8..41a3ff4 100644
--- a/vmbase/example/src/pci.rs
+++ b/vmbase/example/src/pci.rs
@@ -98,7 +98,7 @@
 
 struct HalImpl;
 
-impl Hal for HalImpl {
+unsafe impl Hal for HalImpl {
     fn dma_alloc(pages: usize, _direction: BufferDirection) -> (PhysAddr, NonNull<u8>) {
         debug!("dma_alloc: pages={}", pages);
         let layout = Layout::from_size_align(pages * PAGE_SIZE, PAGE_SIZE).unwrap();
@@ -110,7 +110,7 @@
         (paddr, vaddr)
     }
 
-    fn dma_dealloc(paddr: PhysAddr, vaddr: NonNull<u8>, pages: usize) -> i32 {
+    unsafe fn dma_dealloc(paddr: PhysAddr, vaddr: NonNull<u8>, pages: usize) -> i32 {
         debug!("dma_dealloc: paddr={:#x}, pages={}", paddr, pages);
         let layout = Layout::from_size_align(pages * PAGE_SIZE, PAGE_SIZE).unwrap();
         // Safe because the memory was allocated by `dma_alloc` above using the same allocator, and
@@ -121,17 +121,17 @@
         0
     }
 
-    fn mmio_phys_to_virt(paddr: PhysAddr, _size: usize) -> NonNull<u8> {
+    unsafe fn mmio_phys_to_virt(paddr: PhysAddr, _size: usize) -> NonNull<u8> {
         NonNull::new(paddr as _).unwrap()
     }
 
-    fn share(buffer: NonNull<[u8]>, _direction: BufferDirection) -> PhysAddr {
+    unsafe fn share(buffer: NonNull<[u8]>, _direction: BufferDirection) -> PhysAddr {
         let vaddr = buffer.cast();
         // Nothing to do, as the host already has access to all memory.
         virt_to_phys(vaddr)
     }
 
-    fn unshare(_paddr: PhysAddr, _buffer: NonNull<[u8]>, _direction: BufferDirection) {
+    unsafe fn unshare(_paddr: PhysAddr, _buffer: NonNull<[u8]>, _direction: BufferDirection) {
         // Nothing to do, as the host already has access to all memory and we didn't copy the buffer
         // anywhere else.
     }