xl.cfg: videoram and stdvga documentation improvements

xl.cfg: videoram and stdvga documentation improvements

- videoram: Document that only qemu-xen-traditional device-model currently
   supports changing the amount of video memory for stdvga graphics device.

- videoram: Better document the default amount of videoram for both stdvga
  and Cirrus.

- stdvga: Add a note that stdvga allows bigger amount of videoram and
  bigger resolutions.

Signed-off-by: Pasi Kärkkäinen <pasik@iki.fi>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>