xenlight: add support for .qcow2 via qdisk

If the VDI attach_info contains "format" information, we
select the QDISK libxl disk backend. This allows a storage
implementation to return "format": "qcow2".

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update to 0.9.18; add ChangeLog entry.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #34 from djs55/add-qcow2

Add qcow2 file format support