test: rework run_qemu

test: rework run_qemu

It tries to find a suitable QEMU binary and will use KVM if present.
We can now configure QEMU from outside with 4 variables :
  - $QEMU_BIN : path to QEMU's binary
  - $KERNEL_APPEND : arguments appended to kernel cmdline
  - $KERNEL_BIN : path to a kernel
    Default /boot/vmlinuz-$KERNEL_VER
  - $INITRD : path to an initramfs
    Default /boot/initramfs-${KERNEL_VER}.img
  - $QEMU_SMP : number of CPU simulated by QEMU.
    Default 1

(from Alexander Graf's script: http://www.spinics.net/lists/kvm/msg72389.html)