SPARC64: use serial console when QEMU is launched with -vga none

If no display device has been installed in the DT e.g. passing -vga none into
the QEMU command line then fall back to using the serial console as already
occurs when launching QEMU in -nographic mode.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>