rust: vmstate: implement Zeroable for VMStateField

This shortens a bit the constants.  Do not bother using it
in the vmstate macros since most of them will go away soon.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/zeroable.rs

@@ -49,6 +49,37 @@ unsafe impl Zeroable for crate::bindings::Property {
     };
 }
 
+// bindgen does not derive Default here
+#[allow(clippy::derivable_impls)]
+impl Default for crate::bindings::VMStateFlags {
+    fn default() -> Self {
+        Self(0)
+    }
+}
+
+unsafe impl Zeroable for crate::bindings::VMStateFlags {
+    const ZERO: Self = Self(0);
+}
+
+unsafe impl Zeroable for crate::bindings::VMStateField {
+    const ZERO: Self = Self {
+        name: ptr::null(),
+        err_hint: ptr::null(),
+        offset: 0,
+        size: 0,
+        start: 0,
+        num: 0,
+        num_offset: 0,
+        size_offset: 0,
+        info: ptr::null(),
+        flags: Zeroable::ZERO,
+        vmsd: ptr::null(),
+        version_id: 0,
+        struct_version_id: 0,
+        field_exists: None,
+    };
+}
+
 unsafe impl Zeroable for crate::bindings::VMStateDescription {
     const ZERO: Self = Self {
         name: ptr::null(),