qemud: send message size in hexadecimal format

All sizes need to be printed in hexadecimal. Otherwise size of the
actual data doesn't match and messages are not processed inside the
container.

src/anbox/qemu/qemud_message_processor.cpp

@@ -71,7 +71,7 @@ void QemudMessageProcessor::process_commands() {
 
 void QemudMessageProcessor::send_header(const size_t &size) {
   char header[header_size + 1];
-  std::snprintf(header, header_size + 1, "%04lu", size);
+  std::snprintf(header, header_size + 1, "%04lx", size);
   messenger_->send(header, header_size);
 }