xenserver: Add monitor-external-ids daemon

xenserver: Add monitor-external-ids daemon

The monitor-external-ids daemon monitors the external_ids columns of the
Bridge and Interface OVSDB tables.  Its primary responsibility is to
set the "bridge-id" and "iface-id" keys in the Bridge and Interface
tables, respectively.  It also looks for the use of "network-uuids" in
the Bridge table and duplicates its value to the preferred
"xs-network-uuids".

Signed-off-by: Justin Pettit <jpettit@nicira.com>