When we migrate, or retype with migration, a volume between availability
zones we are setting the AZ of the migrated volume to the same AZ as the
source volume, so we end up with a volume where the AZ does not match
the backend's AZ.
This patch fixes this issue for the generic migration code as well as
the optimized driver migration.
Change-Id: Ia1bf3ae0b7cd6d209354024848aef6029179d8e4
Closes-Bug: #1747949
