Up until now we would really only care about the size of ephemeral and
swap BDM entries'volume_size' being set properly, while we would rely on
Cinder to be the canonical source of truth for the size information of
snapshots and volumes.
There is some overhead in querying Cinder every time this basic
information is needed, and we do have a volume_size field in all BDM
entries already, so let's keep it up to date.
Change-Id: I9ffbb6cf40c1012f774a382cf1dcceba64637e6d
Related-bug: #1370177