Beautify disk ipolicy violations in cluster-verify

Currently, we only get:

  instance3: ['disk-size value 512 is not in range [1024, 1048576]'

which doesn't explain which disk we are talking about. This patch
extends the verification functions to take an additional parameter
that qualifies the disk:

  instance3: ['disk-size/0 value 512 is not in range [1024, 1048576]'

Future patch will make the formatting of the list better.

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