libxl: fix qemu-trad cmdline for no sdl/vnc case

libxl: fix qemu-trad cmdline for no sdl/vnc case

When qemu is running in stubdomain, any attempt to initialize vnc/sdl
there will crash it (on failed attempt to load a keymap from a file). If
vfb is present, all those cases are skipped. But since
b053f0c4c9e533f3d97837cf897eb920b8355ed3 "libxl: do not start dom0 qemu
for stubdomain when not needed" it is possible to create a stubdomain
without vfb and contrary to the comment -vnc none do trigger VNC
initialization code (just skips exposing it externally).
Change the implicit SDL avoiding method to -nographics option, used when
none of SDL or VNC is enabled.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>