Make instance_get_all_*() funtions support the smart extra.$foo columns

This is a follow-on to a previous patch set that made the extra columns
valid for being specified in columns_to_join, as they require slightly
different handling.

Related to blueprint flavor-from-sysmeta-to-blob

Change-Id: Ieffec0e846565b7521fe620694d9f84c67066e11
Merge "Make instance_get_all_*() funtions support the smart extra.$foo columns"