mount: Handle deleted bindmounts

To handle deleted bindmounts we simply create
the former directory bindmount lived at, mount
the target and remove the directory back.

For this sake we add @deleted entry into the image.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>