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
Make xenapi use Instance object for host_maintenance_mode()

This makes xenapi grab an Instance object in host_maintenance_mode()
instead of calling into virtapi. This removes the one and only use
of instance_get_by_uuid() in nova/virt.

The existing tests for host_maintenance_mode() need no changes
since they just stub out the database call that eventually gets
called in the same way.

Related to blueprint unified-object-model

Change-Id: Iff4b5de1741653fad8c59e311341aeb3d0bb3539
Merge "Make xenapi/host use instance objects for _uuid_find"
Merge "Make xenapi use Instance object for host_maintenance_mode()"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp