Fix the virtual_libraries lazy getter

Merge branch 'master' of https://github.com/cbhaley/calibre
newdb: Fix sorting of is_multiple fields

Fix sorting of book list by multi-valued fields like tags not correct in
the new backend. Fixes #1215820 [different ordering by tags in the 1.0 version](https://bugs.launchpad.net/calibre/+bug/1215820)
Fix (I think) for bug #1215885 - refresh rows when convert finishes so composites can update.