Add DeviceSet interface

This interface matches the device-mapper implementation (DeviceSetDM)
but is free from any dependencies. This allows core docker code
to refer to a DeviceSet without having an explicit dependency on
the devmapper package.

This is important, because the devmapper package has external
dependencies which are not wanted in the docker client app, as it
needs to run with minimal dependencies in the docker image.

deviceset.go

@@ -0,0 +1,11 @@
+package docker
+
+type DeviceSet interface {
+	AddDevice(hash, baseHash string) error
+	SetInitialized(hash string) error
+	DeactivateDevice(hash string) error
+	RemoveDevice(hash string) error
+	MountDevice(hash, path string) error
+	HasDevice(hash string) bool
+	HasInitializedDevice(hash string) bool
+}