vbguess: Don't modify guess information on new config

Guess information should be set only when an index is known to be bad.
This commit also adds some logging

Change-Id: Icc366ad4154249342e4bc55614d1919fddf842cf
Reviewed-on: http://review.couchbase.org/43530
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>