Make xenapi/host use instance objects for _uuid_find

This is the one and only use of instance_get_all_by_host() in the
nova/virt tree. Converting it to objects immediately eliminates the
need for that method in the virtapi.

Related to blueprint unified-object-model

Change-Id: I21dfd5a34fb1eef4d442c0711f37e5ef6b18e5bb
Merge "Make xenapi/host use instance objects for _uuid_find"