arm: launch: libvirt: Don't try setting a cpu model on ARM.

One day we may be able to make this work, but presently libvirt says:

  CPU specification not supported by hypervisor
arm: launch: libvirt: Use vexpress-a9 machine type.
arm: launch: libvirt: Force <emulator>.

This is a temporary hack, because otherwise libvirt tries to use
/usr/bin/qemu-kvm even though this is ARM.
arm: launch: libvirt: Set kernel_irqchip=off for KVM to work around buggy qemu/kernel.