xenserver: Fix up iface-id after it changes or disappears too.

ovs-xapi-sync is supposed to always keep external-ids:iface-id up to date,
but in fact it would only set it when an interface initially appeared.  If
the interface quickly disappeared and reappeared, then it failed to notice
that iface-id had changed or disappeared.  This happens in practice on
Citrix XenServer, where VM "tap" devices often disappear and then reappear
almost immediately during VM boot.  This commit fixes the problem.

This also fixes the similar problem for external-ids:bridge-id in Bridge
records.  Bridges aren't ordinarily destroyed and re-created quickly, so
this problem might never have manifested in practice for bridges.

Many thanks to Reid Price <reid@nicira.com> for identifying the problem
and supplying an initial fix.

Bug #5239.
Reported-by: Henrik Amren <henrik@nicira.com>