configs/qemu: update arm-vexpress instructions for SMP

We can use the vexpress arm defconfig to test SMP features in Qemu.
So document the necessary invocation changes to account for this, in
preparation of the arm nuri defconfig removal.

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