Use model_query() instead of session.query in db.instance_destroy

Unify db.instance_destroy() method to use model_query() as others
methods use.

Side effect we won't try to soft delete already soft deleted instance

Change-Id: I36d5bb42b16901c260ff4276e31a6aec46ffa8a4
Improved debug ability for log message of cold migration

When cold migration failed because of nova scheduler return no
valid host, conductor only log no valid host, but did not specify
which instance failed.

This patch was specifying instance in the failure log for cold
migration.

Change-Id: Ia9e42f8f253323fb35a58e0be2a24ab043204d29
Merge "Use model_query() instead of session.query in db.instance_destroy"
Merge "Improved debug ability for log message of cold migration"