Storage backend should be able to recommend a 'backend' kind

[djs55]

It should be the job of the storage system to recommend a particular backend implementation, whether this is

• vbd: blkback
• vbd3: tapdisk3
• qdisk: qemu qdisk

For example, a Ceph implementation could recommend 'qdisk'; a regular .vhd backend could recommend 'vbd3' and an LVM implementation with type=raw could recommend 'vbd'.

It's fine to keep a per-VBD override for people who really know what they're doing.

[djs55]

A simple implementation would allow the storage layer to provide xenstore_data:backend-kind= -- this is how the current per-VBD override works.