CBQE:0 Correct diag eval to enable xdcr trace logging

Change-Id: I68fc2068d84839427fc24c4710195a1fa5df6744
Reviewed-on: http://review.couchbase.org/38724
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Aruna Piravi <aruna@couchbase.com>