HAproxy: change Spice options

- Add "option" parameter to binding function
- Use this new parameter for adding options to Spice binding
  configuration:
  - Use "leastconn" mode instead of "roundrobin"
  - Use a 2 hours timeout to avoid Spice disconnection after some idle
    activity both on client & server sides.

Closes-bug #463
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>