xenserverd: Give XAPI a grace period before refreshing network UUIDs.

xenserverd: Give XAPI a grace period before refreshing network UUIDs.

XAPI updates the pool.conf file before it actually refreshes its database
from the new pool, so we need to wait for that to happen.  Hard-coded
delays aren't a good idea, but in the long term XAPI will probably be
adding a hook script for us to use, so this may be an OK stopgap measure.

Bug #2756.