fix new 1.81.0 warning and clippy error (#760)

* fix new 1.81.0 warning and clippy error

on 1.81, this expression caused rustc warning, and clippy error:

```
warning: this function depends on never type fallback being `()`
   --> bin/propolis-cli/src/main.rs:497:1
    |
497 | / async fn migrate_instance(
498 | |     src_client: Client,
499 | |     dst_client: Client,
500 | |     src_addr: SocketAddr,
501 | |     dst_uuid: Uuid,
502 | |     disks: Vec<DiskRequest>,
503 | | ) -> anyhow::Result<()> {
    | |_______________________^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #123748 <rust-lang/rust#123748>
    = help: specify the types explicitly
note: in edition 2024, the requirement `!: FromIterator<()>` will fail
   --> bin/propolis-cli/src/main.rs:598:20
    |
598 |         .collect::<anyhow::Result<_>>()?;
    |                    ^^^^^^^^^^^^^^^^^
    = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default

warning: `propolis-cli` (bin "propolis-cli") generated 1 warning
```

i am, honestly, entirely missing how the never type is involved here, or how
changing the `collect`'s type from `anyhow::Result<_>` to `anyhow::Result<()>`
changes things in a direction to satisfy rustc. but bounding the type further
doesn't seem like it would cause a problem?

* the unused pub(crate) struct warnings are new also, allow these cases

Author: iximeow

bin/propolis-cli/src/main.rs

@@ -595,7 +595,7 @@ async fn migrate_instance(
         .collect::<Result<Vec<_>, _>>()?
         // Then any errors from polling the source/destination
         .into_iter()
-        .collect::<anyhow::Result<_>>()?;
+        .collect::<anyhow::Result<()>>()?;
 
     Ok(())
 }

bin/propolis-utils/src/bin/cpuid-gen.rs

@@ -16,17 +16,23 @@ fn create_vm() -> anyhow::Result<VmmFd> {
     let ctl = VmmCtlFd::open()?;
     let _ = unsafe { ctl.ioctl(bhyve_api::VMM_CREATE_VM, &mut req) }?;
 
-    let vm = VmmFd::open(&name).map_err(|e| {
-        // Attempt to manually destroy the VM if we cannot open it
-        let _ = ctl.vm_destroy(name.as_bytes());
-        e
-    })?;
-
-    vm.ioctl_usize(bhyve_api::ioctls::VM_SET_AUTODESTRUCT, 1).map_err(|e| {
-        // Destroy instance if auto-destruct cannot be set
-        let _ = vm.ioctl_usize(bhyve_api::VM_DESTROY_SELF, 0);
-        e
-    })?;
+    let vm = match VmmFd::open(&name) {
+        Ok(vm) => vm,
+        Err(e) => {
+            // Attempt to manually destroy the VM if we cannot open it
+            let _ = ctl.vm_destroy(name.as_bytes());
+            return Err(e.into());
+        }
+    };
+
+    match vm.ioctl_usize(bhyve_api::ioctls::VM_SET_AUTODESTRUCT, 1) {
+        Ok(_res) => {}
+        Err(e) => {
+            // Destroy instance if auto-destruct cannot be set
+            let _ = vm.ioctl_usize(bhyve_api::VM_DESTROY_SELF, 0);
+            return Err(e.into());
+        }
+    };
 
     Ok(vm)
 }

lib/propolis/src/firmware/smbios/bits.rs

@@ -140,9 +140,11 @@ pub(crate) struct Type1 {
 }
 raw_table_impl!(Type1, 1);
 
+// We don't create type 2 tables yet, but it's nice to have the definition on hand.
 /// Type 2 (Baseboard Information)
 #[derive(Copy, Clone)]
 #[repr(C, packed)]
+#[allow(dead_code)]
 pub(crate) struct Type2 {
     pub header: StructureHeader,
     pub manufacturer: u8,
@@ -159,9 +161,11 @@ pub(crate) struct Type2 {
 }
 raw_table_impl!(Type2, 2);
 
+// We don't create type 3 tables yet, but it's nice to have the definition on hand.
 /// Type 3 (System Enclosure) - Version 2.7
 #[derive(Copy, Clone)]
 #[repr(C, packed)]
+#[allow(dead_code)]
 pub(crate) struct Type3 {
     pub header: StructureHeader,
     pub manufacturer: u8,