virtio-devices: iommu: Clamp bypass config field to 0 or 1

The virtio spec says the device must never present a value other
than 0 or 1 for bypass. Mask off the upper bits on write so a
later read does not return whatever the driver wrote.

Signed-off-by: Rob Bradford <rbradford@meta.com>
Assisted-by: Claude:claude-opus-4-7

virtio-devices/src/iommu.rs

@@ -1241,7 +1241,9 @@ impl VirtioDevice for Iommu {
             return;
         }
 
-        self.config.bypass = data[0];
+        // virtio spec says ignore bits 1-7 and that the device must
+        // never present a value other than 0 or 1.
+        self.config.bypass = data[0] & 1;
 
         self.update_bypass();
     }