Provide an accessor function serial_hd() to return the Chardev
(if any) associated with the numbered serial port. This will
be used to replace direct accesses to the serial_hds[] array,
so that calling code doesn't need to care about the size of
that array.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420145249.32435-7-peter.maydell@linaro.org