Read flavor even if it is already deleted

Currently libvirt code has some references to Flavor
as objects.Flavor.get_by_id. because the flavor might be
deleted before it's referenced, so most code need to
read deleted flavor in libvirt driver. This patch
makes the _prepare_args_for_get_config which is used for
sriov feature consistent to other Flavor get functions.

Related-Bug: #1352668

Change-Id: Ia4a03467720f1fed18cabe1b78bdd4488f316e34
Merge "Read flavor even if it is already deleted"