mount: Dump/restore devtmpfs if it's virtualized

In case if we meet virtualized devtmpfs on dump
(which means its s_dev is different from one obtained
 during mountpoints dump procedure) we should dump it
with tar help. Thus on restore it get filled from the
image.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>