Set default console to virtio on S390x

Set default console to virtio on S390x

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.

Add flags to QEMUMachine to indicate which kind of default devices make
sense for the machine in question.  Use it for S390x: enable virtcon,
disable serial, parallel and vga.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>