Implement network locking in Instance queries

This is needed in case more info than each nic's network uuid is to
be returned. We need to lock networks to get valid data. For now
only the name is returned as an extra field. All other can be added
with trivial effort.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>