Make default vbd-backend vbd3
Make default_vbd_backend_kind configurable option

This can be set by specifying default-vbd-backend-kind in xenopsd.conf.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-6172: Remove baked-in vbd XenStore paths
Make default VBD backend 'vbd'
Merge pull request #55 from simonjbeaumont/ea-1158-tr3

Configurable VBD backends