ovs-xapi-sync: Always set iface-id, not just when xs-vif-uuid changes.

ovs-xapi-sync: Always set iface-id, not just when xs-vif-uuid changes.

When XAPI moves an interface from one bridge to another, the vif script
removes the vif from one bridge and adds it to (possibly) a different
bridge in a single transaction.  The new record does not have an iface-id
initially (because the vif script never adds the iface-id initially) but
it has the same name and xs-vif-uuid as the old one, so the caching logic
in ovs-xapi-sync failed to add a new iface-id.  This commit fixes the
caching logic.

Observed on XenServer 5.6.100.  It's possible that XAPI behavior changed in
later versions so the bug cannot be triggered there, but we have not
checked.

Bug #9414.
Reported-by: Duffie Cooley <dcooley@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>