virtio-netd: replace unimplemented!() with random MAC fallback

When the VIRTIO device doesn't report a MAC in config space,
generate a random locally-administered unicast MAC instead of
panicking with unimplemented!(). This matches Linux 7.1
drivers/net/virtio_net.c virtnet_probe() behavior.

Reads /scheme/rand for random bytes; falls back to a fixed
MAC if the random source is unavailable. MAC is forced to
unicast + locally administered (bit 0=0, bit 1=1).

drivers/net/virtio-netd/src/main.rs

@@ -84,7 +84,21 @@ fn deamon(
         device.transport.ack_driver_feature(VIRTIO_NET_F_MAC);
         mac
     } else {
-        unimplemented!()
+        // VIRTIO device didn't report a MAC in config space.
+        // Generate a random locally-administered unicast MAC as fallback.
+        // Linux 7.1 drivers/net/virtio_net.c: virtnet_probe() also
+        // generates a random MAC when the device doesn't provide one.
+        let mut mac = [0u8; 6];
+        if let Ok(rand_bytes) = std::fs::read("/scheme/rand") {
+            let n = rand_bytes.len().min(6);
+            mac[..n].copy_from_slice(&rand_bytes[..n]);
+        } else {
+            // Fallback: use a fixed locally-administered MAC
+            mac = [0x02, 0x00, 0x00, 0x00, 0x00, 0x01];
+        }
+        mac[0] = (mac[0] & 0xFE) | 0x02; // unicast + locally administered
+        log::warn!("virtio-netd: no MAC in config, using random {:02x?}", mac);
+        mac
     };
 
     device.transport.finalize_features();