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 "Improved debug ability for log message of cold migration"