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 <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-id: 20180420145249.32435-2-peter.maydell@linaro.org