xenserver: make ovs-xenserverd startup configurable and disable it for 5.6.0 onwards.

xenserver: make ovs-xenserverd startup configurable and disable it for 5.6.0 onwards.

In my testing with XenServer 5.6.810 without ovs-xenserverd the
vswitch bridge external-id network-uuid field was always kept up to
date over both pool join and pool eject. I believe this is because
xapi is restarted on both pool join and pool eject and on restart all
PIFs are plugged again. This causes the vswitch field to be updated,
either via a call to interface reconfigure or via an explicit
ovs-vsctl call in the case of internal networks.

I think the only reason this daemon would still be required with
XenServer 5.5 is that the explicit call to ovs-vsctl for internl
networks is not present there.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>