xenserver: Update network UUIDs whenever XAPI restarts.

xenserver: Update network UUIDs whenever XAPI restarts.

XAPI calls the "update" method of the vswitch-cfg-update plugin whenever it
starts or restarts.  Thus, this is a good place to trigger updating network
UUIDs.  This commit implements that behavior.  This should fix the problem
of network UUIDs not being updated on pool join, since XAPI restarts when
it joins a pool.

When XenServer 5.5 support is no longer interesting, we should integrate
the refresh-network-uuids script into the XAPI plugin.

Lightly tested, works for me on 5.6.810-31078p.

See-also: http://openvswitch.org/pipermail/dev_openvswitch.org/2010-June/002216.html
Suggested-by: Rob Hoes <rob.hoes@citrix.com>
Bug #2097.