hw/usb/dev-serial: Explicit we ignore few QEMUChrEvent in IOEventHandler

hw/usb/dev-serial: Explicit we ignore few QEMUChrEvent in IOEventHandler

The Chardev events are listed in the QEMUChrEvent enum. To be
able to use this enum in the IOEventHandler typedef, we need to
explicit all the events ignored by this frontend, to silent the
following GCC warning:

  hw/usb/dev-serial.c: In function ‘usb_serial_event’:
  hw/usb/dev-serial.c:468:5: error: enumeration value ‘CHR_EVENT_MUX_IN’ not handled in switch [-Werror=switch]
    468 |     switch (event) {
        |     ^~~~~~
  hw/usb/dev-serial.c:468:5: error: enumeration value ‘CHR_EVENT_MUX_OUT’ not handled in switch [-Werror=switch]
  cc1: all warnings being treated as errors

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20191218172009.8868-5-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>