rust/vmstate: Fix type check for varray in vmstate_struct

When pass a varray to vmstate_struct, the `type` parameter should be the
type of the element in the varray, for example:

vmstate_struct!(HPETState, timers, [0 .. num_timers], VMSTATE_HPET_TIMER,
		BqlRefCell<HPETTimer>).with_version_id(0)

But this breaks current type check, because it checks the type of
`field`, which is an array type (for the above example, type of timers
is [BqlRefCell<HPETTimer>; 32], not BqlRefCell<HPETTimer>).

But the current assert_field_type() can no longer be extended to include
new arguments, so a variant of it (a second macro containing the
`num = $num:ident` parameter) had to be added to handle array cases.

In this new macro, it not only checks the type of element, but also
checks whether the `num` (number of elements in varray) is out of range.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250318130219.1799170-6-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/assertions.rs

@@ -91,6 +91,21 @@ macro_rules! assert_field_type {
             }
         };
     };
+
+    ($t:ty, $i:tt, $ti:ty, num = $num:ident) => {
+        const _: () = {
+            #[allow(unused)]
+            fn assert_field_type(v: $t) {
+                fn types_must_be_equal<T, U>(_: T)
+                where
+                    T: $crate::assertions::EqType<Itself = U>,
+                {
+                }
+                let index: usize = v.$num.try_into().unwrap();
+                types_must_be_equal::<_, &$ti>(&v.$i[index]);
+            }
+        };
+    };
 }
 
 /// Assert that an expression matches a pattern.  This can also be