vl.c: Provide accessor function serial_hd() for serial_hds[] array

vl.c: Provide accessor function serial_hd() for serial_hds[] array

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 <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20180420145249[email protected]