Confuse error message when KVM can not start requested CPU

As of commit 1ba089f2255, on Cavium CN8890 (ThunderX cores):

$ qemu-system-aarch64 -display none -accel kvm -M virt,gic-version=3 -accel kvm -cpu cortex-a57 --trace \*kvm_vcpu\*      
kvm_vcpu_ioctl cpu_index 0, type 0x4020aeae, arg 0xffff9b7f9b18 
qemu-system-aarch64: kvm_init_vcpu: kvm_arch_init_vcpu failed (0): Invalid argument

(same using "-cpu cortex-a53" or cortex-a72).

Explanation from Peter Maydell on IRC:
> using a specific cpu type will only work with KVM if the host CPU really is that
> exact CPU type, otherwise, use "-cpu host" or "-cpu max"

Having a better error description would help to understand the reason.