This patch adds VIR_MIGRATE_UNSAFE flag for migration APIs and new
VIR_ERR_MIGRATION_UNSAFE error code.  The error code should be returned
whenever migrating a domain is considered unsafe (e.g., it's configured
in a way that does not ensure data integrity once it is migrated).
VIR_MIGRATE_UNSAFE flag may be used to force migration even though it
would normally be considered unsafe and forbidden.