block: add sheepdog driver for distributed storage support

block: add sheepdog driver for distributed storage support

Sheepdog is a distributed storage system for QEMU. It provides highly
available block level storage volumes to VMs like Amazon EBS.  This
patch adds a qemu block driver for Sheepdog.

Sheepdog features are:
- No node in the cluster is special (no metadata node, no control
  node, etc)
- Linear scalability in performance and capacity
- No single point of failure
- Autonomous management (zero configuration)
- Useful volume management support such as snapshot and cloning
- Thin provisioning
- Autonomous load balancing

The more details are available at the project site:
    http://www.osrg.net/sheepdog/

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>