arm: trying to use KVM with an EL3-enabled CPU hits an assertion failure

$ qemu-system-aarch64  -enable-kvm -display none -cpu max -machine raspi3b
qemu-system-aarch64: ../../softmmu/physmem.c:747: cpu_address_space_init: Assertion `asidx == 0 || !kvm_enabled()' failed.
Aborted

The combination won't work, but we should make it fail 'realize' with a helpful error message.

This doesn't affect 'virt' because that board currently has an explicit check for KVM+EL3, but we might be able to remove that in favour of the generic message.