hw/sparc64: Explicitly set default_display = "std"

hw/sparc64: Explicitly set default_display = "std"

The sun4uv_init() function expects vga_interface_type to be either
VGA_STD or VGA_NONE and sets up a stdvga device or no vga card
accordingly.
However, the code in vl.c prefers the Cirrus VGA card to stdvga if
it is available and the user and the machine did not specify anything
else.
So far this has not been a problem, since the Cirrus VGA was not linked
into the sparc64 target. But with the upcoming Kconfig build system,
all theoretically possible PCI cards will be enabled by default, so the
Cirrus VGA card might become available on the sparc64 target, too. vl.c
then picks the wrong card, causing sun4uv_init() to abort.
Thus let's make it explicit that we always want stdvga for sparc64 and
so set default_display = "std" for these machines.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <1550041639-10232-1-git-send-email-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>