How to solicit feedback from students about faculty candidates?
As someone who was a graduate student until recently, I would have no idea how to respond to a request like this. Even if I were qualified to judge potential faculty hires (which I very much am not, even now, as a postdoc) you've simply asked for "feedback" without giving any specifics. Do you know what kind of feedback you're looking for? You have to answer that question for yourself first.

It sounds like this task was thrust on you. Can you get anything more specific from whoever put you in charge of this? 

This whole thing sounds odd to me. In fact, I suspect it's really all for appearances, to make the grad students feel included or to appease some high-level administrator who thinks it's a good idea. If you think that's the case, my advice is to provide free food for grad students after the job talks (without the free food, no one will stay) and hand out evaluation forms with a few questions about the content/style of the talk. The forms are unlikely to be read by anyone, but you'll accomplish the task you were given and the students will get free food. Everyone wins.