hw/char/serial: Allow disconnected chardevs

hw/char/serial: Allow disconnected chardevs

Currently the serial.c realize code has an explicit check that it is not
connected to a disconnected backend (ie one with a NULL chardev).
This isn't what we want -- you should be able to create a serial device
even if it isn't attached to anything. Remove the check.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Acked-by: Thomas Huth <[email protected]>
Message-id: 20180420145249[email protected]