libxl: make it possible to disable vnc

libxl: make it possible to disable vnc

tools/libxl/libxl__build_device_model_args_new/old: The condition is
so rigorous that user has no chance to disable the vnc,
considering what has been done in parse_config_data() by default,
which is not resonable with vnc option in vm-cfg file.

I think, If user explicitly set "vnc=0", vnc should be disabled.
User should have the chance to only use sdl, other remote
display(spice) and even nothing.

Signed-off-by: Zhou Peng <zhoupeng@nfs.iscas.ac.cn>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>