qemu: caps: Tweak arm conditional in SupportsChardev
    
    Rather than try to whitelist all device configs that can't use
    -chardev, blacklist the only one that really can't, which is the
    default serial/console target type=isa case.
    
    ISA specifically isn't a valid config for arm/aarch64, but we've
    always implicitly treated it to mean 'default platform device'.
    
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Signed-off-by: Cole Robinson <crobinso@redhat.com>