Add get_by_network() to FixedIPList

This adds support for get_by_network() to FixedIPList, which is needed
by the linux_net module to go full-objects. Since it depends on
information in the VirtualInterface object, this also adds a nested
FixedIP.virtual_interface object, which currently is only reachable
when queried in this way.

Related to blueprint nova-network-objects

Change-Id: I6b53e6eed1dee049a51d95f385132fc28c7cce05
Related-bug: 1290568
Merge "Add get_by_network() to FixedIPList"