msg_buf_size is unsigned long not size_t

This fixes the build on 32bit systems which otherwise fails with:

virnetmessagetest.c: In function 'testMessageHeaderEncode':
virnetmessagetest.c:75:9: error: format '%zu' expects argument of type 'size_t', but argument 7 has type 'long unsigned int' [-Werror=format]

tests/virnetmessagetest.c

@@ -72,7 +72,7 @@ static int testMessageHeaderEncode(const void *args ATTRIBUTE_UNUSED)
     }
 
     if (msg->bufferLength != msg_buf_size) {
-        VIR_DEBUG("Expect message offset %zu got %zu",
+        VIR_DEBUG("Expect message offset %lu got %zu",
                   msg_buf_size, msg->bufferLength);
         goto cleanup;
     }