[SyliusWebBundle] Fix wrong volume based price form display on product

Hi,

There's some errors in the error :
- The table overflow fields because it is not wrapped with <div> tag
- The form is not diplaying because you use the trim funtion in the data-prototype call
![bug-volume-based](https://cloud.githubusercontent.com/assets/1230954/4149511/bbc2dd32-3434-11e4-8f6c-d701317aeb00.png)

Here's the fix

![fix-volume-based](https://cloud.githubusercontent.com/assets/1230954/4150189/9f194516-343b-11e4-93d2-a568a0ac9661.png)