bug #891267 : for XS, grab iface-id from XAPI directly if needed.
The version of OVS that ships with XenServer by default does not include
the script to automatically sync the XAPI other_config:nicira-iface-id
field of a VIF with the external_ids:iface-id in the OVS interfaces table.
Thus, make the agent grab the value directly from XAPI if iface-id is
not already populated.