qemu: output error when try to hotplug unsupported console type
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1164627
    
    When using 'virsh attach-device' to hotplug an unsupported console type
    into a qemu guest the attachment would succeed as the command line
    formatter didn't report error in such case.
    
    Signed-off-by: Luyao Huang <lhuang@redhat.com>