Wipe added space when growing disks

This patch adds code to wipe newly added disk space when growing disks
using “gnt-instance grow-disk”. “New disk space” is defined as the delta
between the old block device size (not necessarily equal to the amount
recorded in the configuration) and the new recorded size. Extra caution
is taken to avoid overwriting existing data. Unit tests are included.

A small typo in gnt-cluster(8) (“it's”) is also fixed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>