xenapi: Speed up tests by not waiting on conductor

Part of testing the xenapi agent code requires creating faults. This is
usually sent to the conductor to add to the database, but no conductor
is running during the tests. This results in waiting for a 15 second
timeout for every test that does this.

Disabling use of the conductor during these tests shaves about 45 seconds
off that is spent waiting on a reply that will never come.

Before:
    test_spawn_agent_upgrade_fails_silently  OK  15.20
    test_spawn_fails_agent_not_implemented   OK  15.18
    test_spawn_fails_with_agent_bad_return   OK  15.17

After:
    test_spawn_agent_upgrade_fails_silently  OK  0.16
    test_spawn_fails_agent_not_implemented   OK  0.18
    test_spawn_fails_with_agent_bad_return   OK  0.18

Change-Id: I31fedf6710aba3778eca694b45aabcffe4d09133
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
Make linux_net use objects for last fixed ip query

This converts the final uses of direct database calls from the
linux_net module by utilizing the calls just added to the
FixedIP object.

Related to blueprint nova-network-objects

Related-bug: 1290568
Change-Id: I58828e9112e43967fe8daa070575ca122041690a
Merge "Add get_by_network() to FixedIPList"
Merge "xenapi: Speed up tests by not waiting on conductor"
Merge "Make linux_net use objects for last fixed ip query"