libxl: Add none to vga parameter

Usage:
  vga="none"

With upstream qemu it was impossible to disable emulated vga,
even with -nographic qemu parameter setted with nographic xl
paramter.

Note:
Tested only with upstream qemu. With the qemu-trad. it probably
needs also the addition of -nographic qemu parameter.
Can someone test it with the traditional or should I do it?

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>