Otherwise, the CHR_EVENT_OPENED event is sent twice: first when the
backend (for example "stdio") is opened, and second after processing
the command line.

The incorrect sending of the event prints the monitor banner when
QEMU is started with "-serial mon:stdio".  This includes the "(qemu)"
prompt; thus the monitor seems to be dead, whereas actually the
active front-end is the serial port.
Reported-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>