qemu: Introduce a new capability for virtio-gpu-ccw
    
    QEMU on S390 (since v2.11) can support virtio-gpu-ccw device.
    Let's introduce a new qemu capability for the device.
    
    Signed-off-by: Farhan Ali <alifm@linux.vnet.ibm.com>
    Signed-off-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>