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.

Changes from v1:
- libxl_dm.c:
 if vga is none:
  - add -vga none paramter
  - do not add -videoram parameter.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
libxl: Fix nographic with upstream qemu

Fix xl nographic parameter with upstream qemu using vga none
instead of -nographic qemu parameter not working anymore and
deprecated.

Changes from v1:
- libxl_create.c:
 - add vga=none if nographic=1 only with upstream qemu
 - move setdefault of nographic up, otherwise xl create will
   fail if nographic xl paramter is not set

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