azure: units fail to attach block storage

[Andrew Wilkins]

I've just deployed a charm with block storage to azure, and the unit never initialises. Looking at the unit logs, I see:

    2015-09-23 05:10:18 ERROR juju.worker.uniter.remotestate watcher.go:81 remote state watcher exited: processing initial storage attachment change: refreshing storage details: cannot determine device path: no serial or persistent device name

Looks like we're not combining information from volume sources and block devices properly.