hw/xtensa/xtfpga.c: Don't create "null" chardevs for serial devices

Following commit 12051d82, UART devices should handle
being passed a NULL pointer chardev, so we don't need to
create "null" backends in board code. Remove the code that
does this and updates serial_hds[].
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420145249.32435-6-peter.maydell@linaro.org

hw/xtensa/xtfpga.c

@@ -278,10 +278,6 @@ static void xtfpga_init(const XtfpgaBoardDesc *board, MachineState *machine)
                 xtensa_get_extint(env, 1), nd_table);
     }
 
-    if (!serial_hds[0]) {
-        serial_hds[0] = qemu_chr_new("serial0", "null");
-    }
-
     serial_mm_init(system_io, 0x0d050020, 2, xtensa_get_extint(env, 0),
             115200, serial_hds[0], DEVICE_NATIVE_ENDIAN);