qemu: Support virtio-mmio transport for virtio on ARM
    
    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.