page-xfer: Add helper for dumping a single hole

[xemul: This is an extract from the patch "page-xfer: dump compatible iovec"
 to make rebase simpler ]

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>