Remove shelling out to mount

Also remove the target from the Mount struct because it should not be
used at all.  The target can be variable and set by a caller, not by the
snapshot drivers.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

snapshot/manager.go

@@ -197,7 +197,6 @@ func (lm *Manager) Prepare(dst, parent string) ([]containerd.Mount, error) {
 		{
 			Type:    "overlay",
 			Source:  "none",
-			Target:  dst,
 			Options: opts,
 		},
 	}, nil