[FIX] point_of_sale: fixed rounding issue for pos order when discount ad...

...ded(maintenance: 615322).

Steps to reproduce the issue:
1) Take any product in PoS interface and clear the price/discount/qty manually,
2) set price=1.19, qty=5 and discount=30%

It will result in showing two different totals in the PoS screen as below,
![pos-product_rounding](https://cloud.githubusercontent.com/assets/7601649/4717802/4966e684-5919-11e4-8370-3dfa31f5c6f3.jpg)

This is caused because rounding(round_pr(...)) is not performed on "qty * unitprice" when calculating the base amount for the total like it is performed in the function "get_display_price(...)" in same file.

This branch fixes this issue, kindly review it.

Thanks.