tests/display-vga: Enable virtio-vga test

tests/display-vga: Enable virtio-vga test

There are some "#ifdef CONFIG_VIRTIO_VGA" in the code here which
do not work as expected: CONFIG_VIRTIO_VGA is a Makefile switch,
but not a CPP macro, so the "guarded" code currently simply never
gets enabled.

So enable this code now unconditionally, with some runtime switches
for the architectures that have the VIRTIO_VGA device enabled by
default. Looking at the other if-statement in the main function here,
it also seems like this test was originally supposed to be running
on "mips" and "alpha", too, so enable it now for these architectures
in the Makefile, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1543492248-28356-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>