Hello, Apart from that, should I change something the make QEMU open in graphics > window instead of monitor window ? >
By default QEMU should open a graphics window on vexpress-15, unless you specify the -nographic option. In any case before trying anything on KVM make sure to test your ARM guest on an x86 machine with TCG. Graphics should work there with no hacks at all. Once you are sure that your kernel/userspace combo is good enough then you can move to KVM on an ARM device. Regards.
