Print devicemapper status details in docker info

This adds a generic Status call in the Driver api and
implements if for the devicemapper backend.

The status is an array of key/value strings rather than a map so that
we can guarantee some static order of the docker info output.

graphdriver/driver.go

@@ -19,6 +19,8 @@ type Driver interface {
 	Get(id string) (dir string, err error)
 	Size(id string) (bytes int64, err error)
 
+	Status() [][2]string
+
 	Cleanup() error
 }
 

graphdriver/dummy/driver.go

@@ -27,6 +27,10 @@ func (d *Driver) String() string {
 	return "dummy"
 }
 
+func (d *Driver) Status() [][2]string {
+	return nil
+}
+
 func (d *Driver) Cleanup() error {
 	return nil
 }