Convert last use of direct database instance fetching from network api

This is the last db.instance_*() call in network/api.py. This patch
converts it to use the Instance object.

Related to blueprint compute-manager-objects-juno

Change-Id: Id7897da6daa3d76a95403466be17f9eaef8b7850
Convert network/api.py uses of vif database functions to objects

This makes network/api.py use the VirtualInterface object instead of direct
database queries. This is the last direct/obvious use of nova.db.* in the
network api.

Related to blueprint compute-manager-objects-juno

Change-Id: Ia3eb1694f3708aeceb4c77876a210a7cb6532be1
Merge "Convert last use of direct database instance fetching from network api"
Merge "Convert network/api.py uses of vif database functions to objects"