It is possible to pre-define a character device with the -chardev option
and reference its id as serial device. The man page does not mention this
feature.

Use case: Use stdio as serial, but do not terminate VM on Ctrl-C
          -chardev stdio,id=mystdio,signal=off -serial chardev:mystdio
Signed-off-by: NIngo van Lil <inguin@gmx.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>