Add missing attributes to xml deserializer for volume request

The v1 and v2 xml deserializers for volume requests are missing
support for the imageRef, snapshot_id and source_volid attributes
resulting in xml format create volume requests which specify any
of those attributes being silently ignored. This fix adds support
to v1 and v2 and extends the volumes unit tests to check for this.
Also includes some minor debug logging enhancements to make this
easier to find in future.

Fixes bug #1188581

Change-Id: Ib661c4a961c57e682e0e4e6db98d863b3a99cf71
Add missing exception from volume/api.py

Add InvalidSourceVolume exception that is used in the volume/api.py
without being defined.

Fix bug 1189987

Change-Id: Ifcea41e882e259416c91c91d9b202b3a939eb12f
Merge "Add missing exception from volume/api.py"
Merge "Add missing attributes to xml deserializer for volume request"