monitor: union for command handlers This commits adds a new union member to mon_cmd_t for command handlers and convert monitor_handle_command() and qemu-monitor.hx to use it. This improves type safety. Patchworks-ID: 35337 Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>