MB-12852: Return correct disk size

The function to return the correct disk size in the `_info` handler
wasn't yet implemented.

Change-Id: I0ffca9b281f9c473628165127709377f8d077cb5
Reviewed-on: http://review.couchbase.org/44106
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>