qemu-vexpress can't be launched on machine with more than 4 cores/threads

qemu-system-arm: Invalid SMP CPUs 16. The max CPUs supported by machine 'vexpress-a9' is 4

vexpress-a9 seems to support a maximum of 4 threads, but pmbootstrap tries to launch qemu-vexpress with all the threads available on the host (in my case 16). This means I can't launch it.