contrib/vhost-user-bridge: Add UDP receive hexdump

vhost-user-bridge debug prints UDP TX hexdumps in its transmit handler,
but does not for receives, even though they are beneficial for testing.
Add an RX hexdump in the receive callback.

To delineate between transmits and receives, also add a debug print
indicating that the program is in the transmit handler.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Yodel Eldar <yodel.eldar@yodel.dev>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260129133049.119829-4-yodel.eldar@yodel.dev>

contrib/vhost-user-bridge/vhost-user-bridge.c

@@ -179,6 +179,9 @@ vubr_handle_tx(VuDev *dev, int qidx)
 
     assert(qidx % 2);
 
+    DPRINT("\n\n   ***   IN UDP TRANSMIT HANDLER    ***\n\n");
+    DPRINT("    hdrlen = %d\n", hdrlen);
+
     for (;;) {
         ssize_t ret;
         unsigned int out_num;
@@ -333,6 +336,10 @@ vubr_backend_recv_cb(int sock, void *ctx)
         };
         ret = RETRY_ON_EINTR(recvmsg(vubr->backend_udp_sock, &msg, 0));
 
+        if (ret > 0 && VHOST_USER_BRIDGE_DEBUG) {
+            iov_hexdump(sg, num, stderr, "RX:", ret);
+        }
+
         if (i == 0) {
             iov_restore_front(elem->in_sg, sg, hdrlen);
         }