files: Don't restore pos when file doesn't support one

It's not quite clean, but on dump non-seekable files dump 0xff...ff into
64-bit pos field of the image. Thus on restore we'd face an error in the
do_open_reg. In order not to have separate helper for non-seekable files
just skip the position restore for such.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>