Add virtual interface before add fixed IP on nova-network

nova allow user to add fixed IP to an instance when the instance
is running. This action will fail due to no virtual interface
will be created before create fixed ip.
TypeError: 'NoneType' object is unsubscriptable will be reported.

Change-Id: I5885806d1965022816633d0105606e7aaf763b3a
Closes-Bug: #1294939
Merge "Add virtual interface before add fixed IP on nova-network"