char: Make -chardev help print to stdout

char: Make -chardev help print to stdout

Command line help explicitly requested by the user should be printed
to stdout, not stderr.  We do elsewhere.  Adjust -chardev to match:
use qemu_printf() instead of error_printf().  Plain printf() would be
wrong because we need to print to the current monitor for "chardev-add
help".

Cc: "Marc-André Lureau" <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20190417190641[email protected]>