Starting with qemu 1.6, the qemu-system-arm vexpress-a9 model has a
hardcoded virtio-mmio transport which enables attaching all virtio
devices.

On the command line, we have to use virtio-XXX-device rather than
virtio-XXX-pci, thankfully s390 already set the precedent here so
it's fairly straight forward.

At the XML level, this adds a new device address type virtio-mmio.
The controller and addressing don't have any subelements at the
moment because we they aren't needed for this usecase, but could
be added later if needed.

Add a test case for an ARM guest with one of every virtio device
enabled.