arm/aarch64 -M virt on KVM doesn't and will never work with standard
VGA card emulation. The recommended method is to use type=virtio, so
let's make it the default for video devices without an explicit type
set by the user.
https://bugzilla.redhat.com/show_bug.cgi?id=1404112
Signed-off-by: Cole Robinson <crobinso@redhat.com>
