Use model_query() in db.*****_destroy

Use model_query() instead of session.query in db.volume_destroy,
db.volume_type_destroy, db.transfer_destroy and db.snapshot_destroy as
other methods use.
model_query function is the query helper which is used in most part of
cinder/db/sqlalchemy/api.py.

Change-Id: I97be252df9d40bb97231e3341d5cedec26af2d93
Closes-Bug: #1256483
Add more logging to migrate_volume_completion

Add a debug message on entering the function, and an info message if
it was called to clean up an error. This is useful because Nova copies
the data of in-use volumes, and if it fails there will only be a
message in the Nova logs.

Change-Id: I546d0bae8639072727ebe23394885d40255ba1d4
Merge "Use model_query() in db.*****_destroy"
Merge "Add more logging to migrate_volume_completion"