virttest.qemu_qtree: Remove the quotations in image name

Corrently the image name will include two quotations in the string.
And this will failed the parse. So remove the quotations to make
it work as expect.
Hi @xutian
Not sure we should do such update in a very basic function. I prefer that we have some basic functions that can give the output directly without any change. Just let the tests decide we want to deal with them.