Update Unstage transaction to undo steps done in Stage

In unstage we now adhere to the transaction (or order of steps)
done in Stage. To enable this we stash the image meta data
into a local file on the staging path for use with unstage
request.

This helps in unmapping a stale map, in case the mount or
other steps in the transaction are complete.

Signed-off-by: ShyamsundarR <srangana@redhat.com>

pkg/rbd/errors.go

@@ -57,3 +57,12 @@ type ErrInvalidVolID struct {
 func (e ErrInvalidVolID) Error() string {
 	return e.err.Error()
 }
+
+// ErrMissingStash is returned when the image metadata stash file is not found
+type ErrMissingStash struct {
+	err error
+}
+
+func (e ErrMissingStash) Error() string {
+	return e.err.Error()
+}