Add host to instance_faults table.

Instances can be rescheduled or migrated to different hosts; so, to make
troubleshooting easier, it's convenient to store the host in which the
failure occured with the instance fault record.

Change-Id: Ib2b9a5bab5d95739b730f0eab15884c2db33a782
Merge "Add host to instance_faults table."