tools/libxl: Improve videoram setting

tools/libxl: Improve videoram setting

- If videoram setting is less than 8 mb shows error and exit.
- Added videoram setting for qemu upstream with cirrus (added in qemu 1.3).
- Updated xl.cfg man.
- Default and minimal videoram changed to 16 mb if stdvga is set and upstream
  qemu is being used. This is required by qemu 1.4 to avoid a xen memory error
  (qemu 1.3 doesn't complain about it, probably buggy).

Signed-off-by: Fabio Fantoni <fabio.fantoni@heliman.it>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>