Storage updates

• support directories full of configuration file fragment overrides
• storage: add required_cluster_stack
• use queue prefix org.xen.xapi instead of org.xen.xcp
• storage: redefine SR.probe
• storage: add clustered and health for mirror monitoring
• storage: plumb through name_label and name_description
• guests: add guest agent licensing interface
• storage: add Copying state for mirrors