Add max_size to direct_fetch
Improve shared storage checks for live migration

Distinguish cases that require shared instance drive and shared libvirt
instance directory. Reflect the fact that RBD backed instances have
shared instance drive (and no shared libvirt instance directory) in the
relevant conditionals.
Fix dictionary update and key issues, update tests
Merge pull request #1 from angdraug/rbd-live-migration-havana-ephemeral-rbd

Support live migration of all types of RBD backed instances