KVM: x86: check validity of argument to KVM_SET_MP_STATE

An invalid argument to KVM_SET_MP_STATE has no effect other than making the
vCPU fail to run at the next KVM_RUN.  Since it is extremely unlikely that
any userspace is relying on it, fail with -EINVAL just like for other
architectures.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>