Downgrade the log level when automatic confirm_resize fails

When CONF.resize_confirm_window is not 0, the unconfirmed migration
will be confirmed automatically after CONF.resize_confirm_window
seconds; it might fail due to reason such as instance is locked etc,
so failed to automatically confirm resize is not unexpected.
Downgrade the log from error to debug and after all, user can
either confirm it later or next automatically confirm might success.
Also modify the format of log as well.

Change-Id: I525c803d5e149c49606bcd460e2abb4c9300454a
Merge "Downgrade the log level when automatic confirm_resize fails"