Fix circular reference error when live migration failed

When unexpected exception raised in live migration, the MigrationError
carry another original exception in it. But when oslo.message
serialize the exception, the circular reference error happended. This
patch just pass the exception as unicode string into MigrationError.

Change-Id: I4e449baae74bd9a15490ae7accbd2103bae90d57
Related-Bug: #1377644
Merge "Fix circular reference error when live migration failed"