configs/qemu: update to the latest kernel/headers versions

Used to test the new default binutils 2.24 (all passed).
Also update the virtex readme since the dtb is in output/images as well.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>