All "normal" system emulation targets in qemu I'm aware of display output
on either VGA or serial output.

Our S390x virtio machine doesn't have such kind of legacy hardware. So
instead we need to default to a virtio console.

I'm not particularly proud of this patch. It would be a lot better to
have something in the machine description that tells us about the default
terminal.
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>