libxl: Add none to vga parameter

Usage:
  vga="none"

Make possible to not have an emulated vga on hvm domUs.

Changes from v2:
- set video_memkb to 0 if vga is none.
- remove a check on one condition no more needed.

Changes from v1:
- libxl_dm.c:
 if vga is none, on qemu traditional:
  - add -vga none parameter.
  - do not add -videoram parameter.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
libxl: Add qxl vga interface support for upstream qemu

Usage:
  vga="qxl"

Changes from v15:
- refresh

Changes from v14:
- refresh
- small code impromevents on libxl_dm.c

Changes from v13:
- refresh
- update qemu parameters (from -vga to -device)

NOTE:
Seems needed other changes/fixes on xen side before
have it full working.
After latest Jan Beulich patches doesn't show any other
visible xen errors (at least I can't see them).

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Signed-off-by: Zhou Peng <zpengxen@gmail.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>