Is 
passive counted as Bonus AD ? And will this Bonus damage be counted for 
new E. which scales with Bonus AD.
BoRk Passive: Basic attacks deal 6% of the target's current Health in bonus physical damage
Edit: Of late, I have seen few yi's build this instead of 
and i wonder if that was a smart choice.
No.
I hope not, that would be bad news.
I think it's bonus on hit damage, rather than bonus to his over all damage. Say you have 100AD with yi and you hit some one, the bonus damage from the 6% health wouldn't raise the AD of yi over 100 but still give him the damage on hit.
I think only items that add flat damage to his stats are counted as bonus ad, could be wrong.
+25 ad is bonus ad ... the other effect is bonus physical damage .. so no
I personally feel that guinsoos is not a very good item on yi, he has no AP ratios and although the passive is over powered, BOTRK synergizes better with yi because he gets the instantaneous on-hit effect with sated. Yi is played (should be) as an assassin to quickly burst down the carries. The auto attacks that are required to proc the guinsoos are too time consuming in my opinion. Guinsoos would be better on champs with scaling AP ratios such as Jax, Xin, etc.
